Age distribution of endogenous retroviruses in the human genome. Age distributions of endogenous retroviruses (ERVs) from the automated re-annotation of three human chromosomes. ERV intra-element LTRs on chromosome Y are significantly more divergent than those on chromosomes 2 and 1. Top axis shows the number of substitutions per kilo base pairs of intra-element LTR alignments. Age estimates obtained with a rate of 2.1 × 103 substitutions per site per million years .