The RiBi-E(CG) regulon occurs only in Myc-bearing holozoan genomes. (A) Specific amino acid residues in holozoan MAX (red), MAD (blue), MNT (pink), and MYC (green) allow identification among the MYC/MAX superfamily bHLH genes (common superfamily residues in yellow and underlined). Only three bHLH genes were found in the choanoflagellate genome of Monosiga brevicollis: Mb-MYC, MbMAX and MbMUSH, corresponding to Myc and Max orthologs, and a distant MITF/USF/SREBP homolog (not shown). No Myc and Max orthologs were found outside of Holozoa. The predicted amino acid sequences of the bHLH regions of the M. brevicollis Myc/Max family of genes are shown aligned to Drosophila, Caenorhabditis, and Nematostella orthologs. (B) The presence of Myc (green filled boxes) is correlated with multiple genomes possessing the E(CG)-RiBi signature (green filled boxes). Other bHLH genes in the Myc superfamily (Max, Mad, Mnt; gray filled boxes) are either not necessary (Mad or Mnt) or insufficient (Max) to explain the occurrence of E(CG) sites in the RiBi regulon. "X" boxes indicate absence of a gene or E(CG) signature as indicated.