The albino deletions identify at least seven functional intervals essential for pre- and postnatal development in the 6- to 10-cM region surrounding thealbino coat color(c = tyrosinase) locus on mouse chromosome 7. Thec 112K deletion identifies a putative thymus functional region not removed by the overlappingc 3H deletion. Cloning thec 3H proximal breakpoint provided a starting point for construction of an 840-kb BAC contig spanning thec 112K andc 3H (D7Ssb3Hp) proximal breakpoints. These breakpoints are separated by 320–350 kb. Thearyl hydrocarbon receptor nuclear translocator-2 (Arnt2)is completely removed by thec 112K deletion and spans 130–170 kb of the interval. AlthoughArnt2is a candidate for the thymus defects inc 112K homozygotes, the possibility that other as yet unidentified genes in thec 112K deletion are responsible for the abnormalities has not been ruled out.Arnt2is a member of the bHLH-PAS (Per, Ahr, Arnt, Sim) family of transcription factors and shares the highest similarity withArnt.The survival ofc 112K homozygotes markedly contrasts the embryonic lethality observed inArnt-deficient embryos and suggests distinct roles for these related transcription factors during embryogenesis.