Genotype determination. Fetus DNA digested withHindIII was analyzed by Southern blot with an EKLF probe that distinguishes between the knockout allele (10-kb fragment detected) and the wild-type allele (6-kb fragment detected). The same blot was reprobed with human LCR sequences that hybridize to a 0.75-kbHindIII fragment from the transgene. The EKLF genotype and the presence (tg) or absence (nt) of the transgene is indicated at the top of each lane.