参考
http://bioinfogeek.over-blog.com/2017/09/gene-expression-units-explained-rpm-rpkm-fpkm-and-tpm.html
https://media.nature.com/original/nature-assets/nature/journal/v490/n7418/extref/nature11450-s1.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5407692/
https://cloud.tencent.com/developer/article/1455921
RPKM：Reads Per Kilobase Million；
103 normalizes for gene length and 106 for sequencing depth factor.
基因长度10的3次方标准化，测序深度10的6次方
FPKM：(Fragments per kilo base per million mapped reads)
区别仅在于，Fragment 与 Read。RPKM针对早期的SE测序，FPKM则是在PE测序上对RPKM的校正。在PE中，一个Fragments测两端，会得到2条Reads，但由于后期质量或比对的过滤，有可能一个Fragments的2条Reads最后只有一条进入最后的表达量分析。总之，对某一对Reads而言，这2条Reads只能算一个Fragments，所以，Fragment的最终数目是Reads的1到2倍之间。
TPM (Transcript per million)
TPM 考虑了基因长度
TPM does not count for total number of mapped reads
TPM proposed as an alternative to RPKM due to inaccuracy in RPKM measurement
(Wagner et al., 2012)
TPM is suitable for sequencing protocols where reads sequencing depends on gene length
RPM (Reads per million mapped reads)
