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A group of radiation-induced perinatal-lethal deletions that include the albino (c) locus on mouse chromosome 7 causes failure of expression of various hepatocyte-specific genes when homozygous ( S. Gluecksohn-Waelsch, 1979 , Cell 16: 225–237). The transcription of such genes could be controlled in trans by a regulatory gene(s) located within the proximal region of the c14CoS deletion. To identify this potential regulatory gene, a microclone library was established from microdissected D and E bands of chromosome 7. Three nonoverlapping microclones (E305, E336B, and E453B) hybridizing with wildtype but not with c 14CoS c 14CoS DNA were isolated. E336B represents a single-copy DNA fragment, whereas E305 and E453B hybridized with 3 and 10 EcoRI DNA restriction fragments, respectively. All fragments map exclusively within the deletion. The microclones hybridized to DNA of viable c 6H c 14CoS deletion heterozygotes but not to DNA of homozygotes for the lethal mutation c10R75M, which belongs to the same complementation group as c14CoS. DNA of viable homozygous mutant c62DSD, which carries a deletion breakpoint proximal to that of c6H, hybridized only with E453B. This microclone identified 6 EcoRI restriction fragments in c 62DSD c 62DSD DNA. The results demonstrate that of the isolated microclones, E453B identifies a locus (D7RT453B) that maps closest to the hsdr-1 (hepatocyte-specific developmental regulation) locus ( S. Gluecksohn-Waelsch, 1989 , Mouse Newslett. 83: 149), which maps between the proximal breakpoints of deletions c10R75M and c62DSD.