For each of four species, the Venn diagram represents the number and overlap of predicted genes encoding ZF-C2H2 (blue), KRAB (yellow), and SCAN (red) domains. Numbers correspond to the number of proteins in each category, e.g. chicken has 25 proteins with both KRAB and ZF-C2H2 domains and 2 proteins with only KRAB domains. SCAN domains are present in the prediction sets of T. rubripes and X. tropicalis, but are not associated with ZF-C2H2 proteins. The KRAB domain is not present in T. rubripes, but is already associated with ZF-C2H2 proteins in X. tropicalis. H. sapiens has broad overlaps in its sets of ZF-C2H2, KRAB and SCAN proteins, a pattern typical of mammals.