外显子的长度。

有几个相关的概念，你可以了解一下。

RPKM: Reads Per Kilobase of exon model per Million mapped reads 

FPKM: Fragments Per Kilobase of exon model per Million mapped fragments

RPM/CPM: Reads/Counts of exon model per Million mapped reads 

TPM：Transcripts Per Kilobase of exon model per Million mapped reads