RFLNB and cystic fibrosis: In a studied population of genetically mixed CF mice differences in allelic ratios at particular loci, between surviving CF mice and mice with the lethal intestinal defect, were used to map cystic fibrosis modifier gene one, Cfm1. Using this approach, we previously identified an X chromosome locus which may influence the survival to weaning of C57BL/6J × BALB/cJ F2 CF mice.