The KRAB repression domain is one of the most widely distributed transcriptional effector domains yet identified, but its mechanism of repression is unknown. We have cloned a corepressor, KAP-1, which associates with the KRAB domain but not with KRAB mutants that have lost repression activity. KAP-1 can enhance KRAB-mediated repression and is a repressor when directly tethered to DNA. KAP-1 contains a RING finger, B boxes, and a PHD finger; the RING-B1-B2 structure is required for KRAB binding and corepression. We propose that KAP-1 may be a universal corepressor for the large family of KRAB domain-containing transcription factors.