diff --git a/src/cljam/io/cram.clj b/src/cljam/io/cram.clj
new file mode 100644
index 00000000..20ba474a
--- /dev/null
+++ b/src/cljam/io/cram.clj
@@ -0,0 +1,330 @@
+(ns cljam.io.cram
+  {:clj-kondo/ignore [:missing-docstring]}
+  (:require [cljam.io.cram.codecs.rans4x8 :as rans]
+            [cljam.io.cram.itf8 :as itf8]
+            [cljam.io.util.byte-buffer :as bb])
+  (:import [java.io ByteArrayInputStream IOException]
+           [java.nio Buffer ByteBuffer ByteOrder]
+           [java.util Arrays]
+           [org.apache.commons.compress.compressors CompressorStreamFactory]))
+
+(def ^:const cram-magic "CRAM")
+
+(defn decode-itf8-array [bb]
+  (let [n (itf8/decode-itf8 bb)]
+    (loop [i n, acc (transient [])]
+      (if (zero? i)
+        (persistent! acc)
+        (recur (dec i) (conj! acc (itf8/decode-itf8 bb)))))))
+
+(defn decode-encoding [^ByteBuffer bb]
+  (let [codec-id (itf8/decode-itf8 bb)
+        _n (itf8/decode-itf8 bb)]
+    (case codec-id
+      0 {:codec :null}
+      1 (let [content-id (itf8/decode-itf8 bb)]
+          {:codec :external
+           :content-id content-id})
+      3 (let [alphabet (decode-itf8-array bb)
+              bit-len (decode-itf8-array bb)]
+          {:codec :huffman, :alphabet alphabet, :bit-len bit-len})
+      4 (let [len-encoding (decode-encoding bb)
+              val-encoding (decode-encoding bb)]
+          {:codec :byte-array-len, :len-encoding len-encoding, :val-encoding val-encoding})
+      5 (let [stop-byte (.get bb)
+              external-id (itf8/decode-itf8 bb)]
+          {:codec :byte-array-stop, :stop-byte stop-byte, :external-id external-id})
+      6 (let [offset (itf8/decode-itf8 bb)
+              length (itf8/decode-itf8 bb)]
+          {:codec :beta, :offset offset, :length length})
+      7 (let [offset (itf8/decode-itf8 bb)
+              k (itf8/decode-itf8 bb)]
+          {:codec :subexp, :offset offset, :k k})
+      9 (let [offset (itf8/decode-itf8 bb)]
+          {:codec :gamma, :offset offset})
+      (throw (ex-info (str "codec " codec-id " not supported") {})))))
+
+(defn decode-file-definition [bb]
+  (when-not (Arrays/equals ^bytes (bb/read-bytes bb 4) (.getBytes cram-magic))
+    (throw (IOException. "Invalid CRAM file")))
+  (let [major (bb/read-ubyte bb)
+        minor (bb/read-ubyte bb)
+        file-id (bb/read-bytes bb 20)]
+    {:version {:major major :minor minor}, :id file-id}))
+
+(defn decode-container-header [^ByteBuffer bb]
+  (let [len (.getInt bb)
+        ref-seq-id (itf8/decode-itf8 bb)
+        start-pos (itf8/decode-itf8 bb)
+        span (itf8/decode-itf8 bb)
+        n-records (itf8/decode-itf8 bb)
+        counter (itf8/decode-ltf8 bb)
+        n-bases (itf8/decode-ltf8 bb)
+        n-blocks (itf8/decode-itf8 bb)
+        landmarks (decode-itf8-array bb)
+        crc (bb/read-bytes bb 4)]
+    {:length len
+     :ref ref-seq-id
+     :start start-pos
+     :span span
+     :records n-records
+     :counter counter
+     :bases n-bases
+     :blocks n-blocks
+     :landmarks landmarks
+     :crc crc}))
+
+(defn split-buffer ^ByteBuffer [^ByteBuffer bb size]
+  (let [^Buffer bb' (.order (.slice bb) ByteOrder/LITTLE_ENDIAN)]
+    (bb/skip bb size)
+    (.limit bb' size)))
+
+(def decode-block-data
+  (let [factory (CompressorStreamFactory.)]
+    (fn [^ByteBuffer bb method size raw-size]
+      ;; TODO: decompress data with specified method
+      (case (long method)
+        0 (split-buffer bb size)
+        4 (->> (split-buffer bb size)
+               rans/decode
+               bb/make-lsb-byte-buffer)
+        (let [compressed (bb/read-bytes bb size)
+              bais (ByteArrayInputStream. compressed)
+              uncompressed (byte-array raw-size)
+              bb' (bb/make-lsb-byte-buffer uncompressed)]
+          (with-open [is (.createCompressorInputStream factory bais)]
+            (.read is uncompressed)
+            bb'))))))
+
+(defn decode-block [bb]
+  (let [method (bb/read-ubyte bb)
+        content-type-id (bb/read-ubyte bb)
+        content-id (itf8/decode-itf8 bb)
+        size (itf8/decode-itf8 bb)
+        raw-size (itf8/decode-itf8 bb)
+        data (decode-block-data bb method size raw-size)
+        crc (bb/read-bytes bb 4)]
+    {:method method
+     :content-type content-type-id
+     :content-id content-id
+     :size size
+     :raw-size raw-size
+     :data data
+     :crc crc}))
+
+(defn decode-cram-header-block [bb]
+  (let [{bb' :data} (decode-block bb)
+        size (bb/read-uint bb')]
+    (bb/read-bytes bb' size)))
+
+(defn decode-tag-dictionary [^ByteBuffer bb]
+  (let [n (itf8/decode-itf8 bb)
+        bb' (split-buffer bb n)
+        decode-tags (fn [bb]
+                      (loop [acc (transient [])]
+                        (let [c1 (long (bb/read-ubyte bb))]
+                          (if (zero? c1)
+                            (persistent! acc)
+                            (let [c2 (bb/read-ubyte bb)
+                                  t (bb/read-ubyte bb)
+                                  tag (keyword (str (char c1) (char c2)))]
+                              (recur (conj! acc {:tag tag, :type (char t)})))))))]
+    (loop [acc (transient [])]
+      (if (.hasRemaining bb')
+        (recur (conj! acc (decode-tags bb')))
+        (persistent! acc)))))
+
+(defn decode-preservation-map [^ByteBuffer bb]
+  (let [_size (itf8/decode-itf8 bb)
+        n (itf8/decode-itf8 bb)]
+    (loop [i n, acc (transient {:RN true, :AP true, :RR true})]
+      (if (zero? i)
+        (persistent! acc)
+        (let [k (keyword (String. ^bytes (bb/read-bytes bb 2)))
+              v (case k
+                  (:RN :AP :RR) (pos? (.get bb))
+                  :SM (bb/read-bytes bb 5)
+                  :TD (decode-tag-dictionary bb))]
+          (recur (dec i) (assoc! acc k v)))))))
+
+(defn decode-data-series-encodings [bb]
+  (let [_size (itf8/decode-itf8 bb)
+        n (itf8/decode-itf8 bb)]
+    (loop [n n, acc (transient {})]
+      (if (zero? n)
+        (persistent! acc)
+        (let [k (keyword (String. ^bytes (bb/read-bytes bb 2)))
+              v (decode-encoding bb)]
+          (recur (dec n) (assoc! acc k v)))))))
+
+(defn decode-tag-encoding-map [bb]
+  (let [_size (itf8/decode-itf8 bb)
+        n (itf8/decode-itf8 bb)]
+    (loop [i n, acc (transient {})]
+      (if (zero? i)
+        (persistent! acc)
+        (let [k (itf8/decode-itf8 bb)
+              c1 (char (bit-and (bit-shift-right k 16) 0xff))
+              c2 (char (bit-and (bit-shift-right k 8) 0xff))
+              t (char (bit-and k 0xff))
+              v (decode-encoding bb)
+              tag (keyword (str c1 c2))]
+          (recur (dec i) (assoc! acc tag {:type t :encoding v})))))))
+
+(defn decode-compression-header-block [bb]
+  (let [{bb' :data} (decode-block bb)
+        preservation-map (decode-preservation-map bb')
+        data-series-encodings (decode-data-series-encodings bb')
+        tag-encoding-map (decode-tag-encoding-map bb')]
+    {:preservation-map preservation-map
+     :data-series data-series-encodings
+     :tags tag-encoding-map}))
+
+(defn decode-slice-header-block [bb]
+  (let [{bb' :data} (decode-block bb)
+        ref-seq-id (itf8/decode-itf8 bb')
+        start (itf8/decode-itf8 bb')
+        span (itf8/decode-itf8 bb')
+        n-records (itf8/decode-itf8 bb')
+        counter (itf8/decode-ltf8 bb')
+        n-blocks (itf8/decode-itf8 bb')
+        content-ids (into [] (map (fn [_] (itf8/decode-itf8 bb')))
+                          (range n-blocks))
+        embedded-reference (itf8/decode-itf8 bb')
+        reference-md5 (bb/read-bytes bb' 16)
+        tags []]
+    {:ref-seq-id ref-seq-id
+     :start start
+     :span span
+     :records n-records
+     :counter counter
+     :blocks n-blocks
+     :content-ids content-ids
+     :embedded-reference embedded-reference
+     :reference-md5 reference-md5
+     :tags tags}))
+
+(defn data-series-type [ds]
+  (case ds
+    (:BF :CF :RI :RL :AP :RG :MF :NS :NP :TS :NF :TL :FN :FP :DL :RS :PD :HC :MQ)
+    :int
+
+    (:FC :BS :BA :QS)
+    :byte
+
+    (:RN :BB :QQ :IN :SC)
+    :bytes))
+
+(defn build-data-series-decoder
+  [{:keys [codec] :as params} data-type content-id->block-data]
+  (case codec
+    :external
+    (let [^ByteBuffer block (get content-id->block-data (:content-id params))]
+      (case data-type
+        :byte #(.get block)
+        :int #(itf8/decode-itf8 block)))
+
+    :huffman
+    (let [{:keys [alphabet bit-len]} params]
+      (assert (and (= (count alphabet) 1)
+                   (zero? (long (first bit-len))))
+              "Huffman coding for more than one word is not supported yet.")
+      (constantly (first alphabet)))
+
+    :byte-array-len
+    (let [{:keys [len-encoding val-encoding]} params
+          len-decoder (build-data-series-decoder len-encoding :int content-id->block-data)
+          val-decoder (build-data-series-decoder val-encoding :byte content-id->block-data)]
+      (fn []
+        (let [len (len-decoder)
+              bb (bb/allocate-lsb-byte-buffer len)]
+          (dotimes [_ len]
+            (.put bb (byte (val-decoder))))
+          (.array bb))))
+
+    :byte-array-stop
+    (let [{:keys [stop-byte external-id]} params
+          ^ByteBuffer block (get content-id->block-data external-id)]
+      (fn []
+        (.mark ^Buffer block)
+        (let [start (.position block)
+              end (do (while (not= (.get block) (byte stop-byte)))
+                      (.position block))
+              len (dec (- end start))]
+          (.reset ^Buffer block)
+          (bb/read-bytes block len))))))
+
+(defn build-data-series-decoders [ds-encodings blocks]
+  (let [content-id->block-data (into {} (map (juxt :content-id :data)) blocks)]
+    (reduce-kv (fn [decoders ds params]
+                 (let [dt (data-series-type ds)
+                       decoder (build-data-series-decoder params dt content-id->block-data)]
+                   (assoc decoders ds decoder)))
+               {} ds-encodings)))
+
+(defn build-positional-data-decoder
+  [_compression-header slice-header {:keys [RI RL AP RG]}]
+  (fn [record]
+    (assoc record
+           :rname (if (= (:ref-seq-id slice-header) -2)
+                    (RI)
+                    (:ref-seq-id slice-header))
+           :len (RL)
+           :pos (AP)
+           :options [{:RG {:type "Z" :value (RG)}}])))
+
+(defn build-read-name-decoder [{:keys [preservation-map]} {:keys [RN]}]
+  (if (:RN preservation-map)
+    #(assoc % :qname (RN))
+    (throw (ex-info "Omitted read names are not supported yet." {}))))
+
+(defn build-read-feature-decoder
+  [{:keys [FN FC FP BA QS BS IN SC HC PD DL RS BB QQ]}]
+  (fn [record]
+    (loop [i (long (FN))
+           acc (transient [])]
+      (if (zero? i)
+        (assoc record :features (persistent! acc))
+        (let [fc (char (FC))
+              fp (FP)
+              data (case fc
+                     \B {:base (BA) :qual (QS)}
+                     \X {:subst (BS)}
+                     \I {:inserted (IN)}
+                     \S {:softclip (SC)}
+                     \H {:hardclip (HC)}
+                     \P {:pad (PD)}
+                     \D {:del (DL)}
+                     \N {:skip (RS)}
+                     \i {:base (BA)}
+                     \b {:bases (BB)}
+                     \q {:quals (QQ)}
+                     \Q {:qual (QS)})]
+          (recur (dec i)
+                 (conj! acc (assoc data :fc fc :fp fp))))))))
+
+(defn build-cram-record-decoder
+  [compression-header slice-header {:keys [BF CF] :as decoders}]
+  (let [pd-decoder (build-positional-data-decoder compression-header
+                                                  slice-header
+                                                  decoders)
+        rn-decoder (build-read-name-decoder compression-header decoders)
+        rf-decoder (build-read-feature-decoder decoders)]
+    (fn []
+      (let [bf (BF)
+            _cf (CF)]
+        (->> {:flag bf}
+             (pd-decoder)
+             (rn-decoder)
+             (rf-decoder))))))
+
+(defn decode-slice-records [compression-header slice-header blocks]
+  (let [decoders (build-data-series-decoders (:data-series compression-header) blocks)
+        record-decoder (build-cram-record-decoder compression-header
+                                                  slice-header
+                                                  decoders)]
+    (letfn [(step [^long i]
+              (when (pos? i)
+                (cons (record-decoder) (lazy-seq (step (dec i))))))]
+      (step (:records slice-header)))))
+