Figure 3.

Excision of the loxP- flanked neomycin cassette. (A) The recombinant allele is shown before and after Cre recombinase-mediated excision. Before excision the endogenous promoter will not regulate neuNT expression because of the poly(A) in the neomycin cassette. To detect removal of the neomycin cassette, the neuNT cDNA was used as a probe in an EcoRI (E) digest. The size of this fragment is depicted in both the excised and nonexcised forms. (B) A Southern blot shows that excision of the loxP-flanked Neo is limited to the spleen, salivary, and mammary glands by the presence of a 4-kb band in those samples. The wild-type allele also is detected by the neuNT cDNA probe, resulting in a faint band of 6.0 kb and a stronger band of 25 kb.