转录组年龄指数，数字越大，转录组越年轻

转录组年轻的意思是：新基因在转录组中的比重越大，这个较大的比重来源于表达的新基因个数增加或者新基因的表达强度增加（或者两者都有）

转录组年轻化（转录组中新基因比重变大）与不同物种相同组织（或者细胞等）的表型差异（分化；创新）有正相关趋势

https://www.nature.com/articles/nature09632

Integrates the age of a gene with its expression level at a given developmental stage and sums this over all genes expressed at the respective stage.

The higher the TAI, the younger the transcriptome.

https://genesdev.cshlp.org/content/31/18/1841

Gene expression data are combined with evolutionary information to convey the relative age of the transcriptome.

The TAI of a given tissue represents a weighted expression metric that gives stronger weight to young phylostrata, compensating for the fact that older phylostrata usually harbor a larger number of genes. A high TAI indicates that the transcriptome is evolutionarily young, while a low TAI indicates that the transcriptome is evolutionarily ancient.