The human protein P1 belongs to a newly discovered class of mammalian nuclear proteins with high sequence homology to yeast replication proteins. We present the entire amino acid sequence of the human protein P1 as predicted from the cDNA sequence, and show that P1 shares three central regions of high sequence similarity (about 75%) and a highly hydrophilic carboxy-terminal region with the yeast Mcm3 replication protein. The human genome most probably contains one P1 gene which is activated when HeLa cells progress to S phase, as shown by a several-fold increase in P1-specific mRNA. However, the amounts of P1 protein do not detectably change during this period, but P1 protein becomes phosphorylated at the beginning of S phase. In contrast to the yeast Mcm proteins, which disappear from nuclei after initiation of DNA replication, protein P1 remains in the nucleus during and after S phase. P1 is dispersed in mitotic cells and may be excluded from binding to chromosomes.