The full-length PRDM9 protein contains four domains: KRAB, SSXRD, PR/SET and ZnF (short for Zinc finger). PRDM9 plays an important role in meiotic recombination but, over the course of evolution, many species have either completely lost this protein (light grey line in the simplified phylogenetic tree to the left), or now carry truncated versions of it (dark grey lines). In species with full-length PRDM9 (top row), recombination hotspots are located at sites where PRDM9 binds to the DNA in the genome, and three residues in the PR/SET domain that are essential for the recombination process (Y276, Y341 and Y357; Wu et al., 2013) are conserved: moreover, the locations of the hotspots change over time due to the rapid evolution of the ZnF domain. In species with truncated PRDM9 (next four rows), recombination hotspots are located at transcription start sites (TSS) or at CpG islands, the three residues in the PR/SET domain are not conserved, and the ZnF domain (if present) does not evolve rapidly. In species that lack PRDM9 (bottom row), recombination hotspots are located at transcription start sites (TSS) or at CpG islands.