Examination of loss of heterozygosity at the Men1 locus in pituitary tumors and livers (as a non-tumorigenic control) from mice with the indicated genotypes. Chromosomal DNA samples prepared from tissues were analyzed by genomic PCR as described in Methods. The wild-type (wt) Men1 allele and the targeted mutant (mut) allele were amplified as the indicated bands on agarose gel electrophoresis.