CXXC finger protein 1 (CFP1) is a component of the Setd1A and Setd1B methyltransferase complexes, localizes to euchromatic regions of the genome, and specifically binds unmethylated CpG dinucleotides in DNA. Murine embryos lacking CFP1 exhibit peri-implantation lethality, a developmental time that correlates with global epigenetic reprogramming. CFP1-deficient embryonic stem (ES) cells exhibit a 70% reduction in global cytosine methylation and a 60% decrease in maintenance DNA methyltransferase (DNMT1) activity. DNMT1 protein level is reduced 50% in CFP1-deficient ES cells. Experiments were performed to investigate the role of CFP1 in regulating maintenance cytosine methylation. Coimmunoprecipitation experiments reveal that endogenous DNMT1 and CFP1 interact in vivo. Protein regions required for the interaction between DNMT1 and CFP1 were mapped. Amino acids 169-493 and 970-1617 of DNMT1 are each sufficient for interaction with CFP1. Three regions spanning the CFP1 protein, amino acids 1-123, 103-367, and 361-656, are each sufficient for interaction with DNMT1. Interestingly, a single-point mutation (C375A) within CFP1 that abolishes the interaction with the Setd1A and Setd1B histone H3K4 methyltransferase complexes does not disrupt the interaction between CFP1 and DNMT1. This result indicates that CFP1 intersects the cytosine methylation machinery independently of its association with the Setd1 complexes. © Copyright 2008, Mary Ann Liebert, Inc.