Two new mouse genes encoding proteins that belong to the yeast minichromosome maintenance (MCM) protein family, which is involved in the initiation of DNA replication, were isolated and their nucleotide sequence was determined. They were a putative CDC46/MCM5 homolog and a putative cdc21 homolog. About 30% amino acid identity was obtained between members in the family, and > 40% between the putative mouse and yeast homologs. The expression of these genes was cell-cycle specific at the late G1 to S phase. Immunochemical analyses showed the physical interaction between mouse P1MCM3 and CDC46 protein. These results suggest that MCM proteins function in co-ordination for DNA replication.