The KRAB (Kru ¨ppel-associated box) domain-containing zincfinger (ZF) genes are part of the poly-ZF family that constitutes the largest mammalian transcriptional regulatory gene family. Almost 50% of transcription factor genes in the human genome encode ZFs, of which 40% contains KRAB domains (1). Typically, KRAB-ZF proteins mediate transcriptional repression yet, to date, little else is known of their function as many KRAB-ZF genes are human-specific, so loss-of-function approaches using gene targeting in mouse embryonic stem cells are not possible