KLF4 is one of the four ‘Yamanaka’ factors that together can endow somatic cells with the potential to adopt a pluripotent embryonic stem cell-like state35 and is highly expressed in the pluripotent progenitors of germ cells, the PGCs.36,37 Its suppression by methylation in non-seminomas might therefore play a key role in their more differentiated and more aggressive state. This evidence concerns the gene KLF4 and seminoma.