The HOX homeodomain is a large family of transcription factors involved in gene regulation, both as transcriptional activators and repressors, at many stages in development. Interactions with other proteins, such as PBC homeodomain proteins, can increase HOX protein affinity and specificity for DNA binding. Shen et al. show that HOX proteins also interact with CBP, a transcriptional regulator with histone acetyltransferase activity. Different members of the HOX family interacted with CBP through the NH2-terminus and the homeodomain, which has a putative consensus acetylation motif. However, the HOX proteins were not acetylated by CBP, instead they inhibited CBP HAT activity in vitro and in vivo. In vivo, this inhibition blocked CBP-mediated increases in reporter gene expression. Addition of CBP to HOX, PBC, and DNA complexes promoted the disassociation of the HOX from DNA based on electrophoretic mobility gel shift assays. Shen et al. suggest that the repressive effects of HOX proteins may be the result of inhibition of CBP HAT activity.