Hypothesis for the evolution of vertebrate RD genes. Features with thick edges represent RD genes interlocked with SUPT3H family members. The closed padlock icon represents the interlocking event between the RUNX2 and SUPT3H, and the open padlock represents the degradation of a copy of SUPT3H, releasing RUNX2 from interlocking. Not enough information is yet available to establish which of the zebrafish RUNX2 genes is interlocked with a SUPT3H gene. "dup": duplication. Top right inset: Hypothesis of how the ancestral RD duplication could map to the first vertebrate tetraploidization event. RT and RX represent the ancestral forms of RUNT and RUNX genes.