RNA velocity—the time derivative of the gene expression state—can be directly estimated by distinguishing between unspliced and spliced mRNAs in common single-cell RNA sequencing protocols. RNA velocity is a high-dimensional vector that predicts the future state of individual cells on a timescale of hours.
The relative abundance of nascent (unspliced) and mature (spliced) mRNA can be exploited to estimate the rates of gene splicing and degradation
这种做轨迹推断的思路不同于之前的看基因表达量的高低次序变化，而是从precursor mRNA、mature mRNA入手。
The balance of unspliced and spliced mRNA abundance is, therefore, an indicator of the future state of mature mRNA abundance, and thus the future state of the cell.
很奇怪吧，为什么内含子也能检测到reads
可能的原因是：对于还不成熟的RNA，内含子里面也含有polyA和polyT序列，这种结构和成熟RNA的polyA相似，也存在被捕获的可能性，后续PCR、测序也能测到。SMART-seq的测序长度比10X的单端测序要长，在图中也能看出来。
参考文献
RNA velocity of single cells
Generalizing RNA velocity to transient cell states through dynamical modeling
