Figure 1: Substitution frequencies along the 5′ UTR sequences of human-specific L1 elements. The percentage of substitutions per nucleotide position along the 5′ UTR sequence was calculated. The structure of the L1 5′ UTR is shown on the top. The TF binding sites, sense promoter (SP), antisense promoter (ASP), CpG sites, and CpG islands are indicated by colored boxes and lines. (a) 443 full-length human-specific L1 elements. (b) 36 active L1 elements in vitro. (c) 41 dead L1 elements in vitro.