CREB-binding protein (CBP) and p300 are highly conserved and functionally related transcription coactivators and histone/protein acetyltransferases. They are tumor suppressors, participate in a wide variety of physiological events, and serve as integrators among different signal transduction pathways. In this study, 11 distinct proteins that have a high degree of homology with the amino acid sequence of p300 have been identified in current protein databases. All of these 11 proteins belong to either animal or plant multicellular organisms (higher eucaryotes). Conservation of p300/CBP domains among these proteins was examined further by sequence alignment and pattern search. The domains of p300/CBP that are required for the HAT function, including PHD, putative CoA-binding, and ZZ domains, are conserved in all of these 11 proteins. This observation is consistent with the previous functional assays and indicates that they are a family of acetyltransferases, i.e. p300/CBP acetyltransferases (PCAT). TAZ domains (TAZ1 and/or TAZ2) of PCAT proteins may allow them to participate in transcription regulation by either directly recruiting transcription factors, acetylating them subsequently, or directing targeted acetylation of nucleosomal histones.