Many of the vertebrate zinc finger factors of the Krüppel type (C2H2 zinc fingers) contain in their N-terminus a conserved sequence referred to as the KRAB (Krüppel-associated box) domain that, when tethered to DNA, efficiently represses transcription. Using the yeast two-hybrid system, we have isolated an 835 amino acid RING finger (C3HC4 zinc finger) protein, TIF1β (also named KAP-1), that specifically interacts with the KRAB domain of the human zinc finger factor KOX1/ZNF10. TIF1β, TIF1α, PML and efp belong to a characteristic subgroup of RING finger proteins that contain one or two other Cys/His-rich clusters (B boxes) and a putative coiled-coil in addition to the classical C3HC4 RING finger motif (RBCC configuration). Like TIF1α, TIF1β also contains an additional Cys/His cluster (PHD finger) and a bromorelated domain. When tethered to DNA, TIF1β can repress transcription in transiently transfected mammalian cells both from promoter-proximal and remote (enhancer) positions, similarly to the KRAB domain itself. We propose that TIF1β is a mediator of the transcriptional repression exerted by the KRAB domain.