Gene expression constraints increase over time, which may be the result of the integration of genes into gene networks and the functional cellular environment. Cis expression quantitative trait loci (eQTLs) of younger genes have a higher effect size, are located nearer to transcription start sites and usually influence genes in more tissues and populations. In addition, the heritability of gene expression increases over time, which indicates that the influence of environment on gene expression levels decreases with gene age.