We have isolated human homologues of the imprinted mouse gene,U2af1-rs1.Two different types of cDNAs and three distinct genomic DNAs belonging to different groups were isolated. We have identified chromosomal genes corresponding to each cDNA by restriction mapping and sequencing. Using both a panel of rodent/human somatic cell hybrids and fluorescencein situhybridization, group 1 and group 2 genes were mapped to chromosome 5q22 and chromosome Xp22.1, respectively. We designated group 1 and group 2 genes as humanU2AF1-RS1andU2AF1-RS2, respectively, because these genes corresponded to mouseU2af1-rs1(chromosome 11) andU2af1-rs2(chromosome X), which we also isolated and mapped. Amino acid sequences of humanU2AF1-RS1andU2AF-RS2showed significant homology to U2AF small subunit. The group 3 gene, designated asU2AF1-RS3,of which the cDNA has not yet been isolated, was mapped to chromosome 19p13.2.