Regulatory T cells (Tregs) play a pivotal role in the maintenance of immunological self-tolerance. Deficiency or dysfunction of Tregs leads to severe autoimmune diseases. Although the forkhead/winged-helix family member FOXP3 is critical for Treg differentiation and function, the molecular basis for FOXP3 function remains unclear. In this study, we identified and characterized a human-specific FOXP3-interacting protein, referred to as FIK (FOXP3-interacting KRAB domain- containing protein). FIK is highly expressed in Tregs and acts as a bridging molecule to link FOXP3 with the chromatin-remodeling scaffold protein KAP1 (TIF-1β/ TRIM28). Disruption of the FOXP3-FIK-KAP1 complex in Tregs restored expression of FOXP3-target genes and abrogated the suppressor activity of the Tregs. These data demonstrate a critical role for FIK in regulating FOXP3 activity and Treg function. Copyright © 2013 by The American Association of Immunologists, Inc.