Lepr^tm1Rck

A single loxP site was inserted 5' to exon 1 and a loxP-flanked neomycin and thymidine kinase selection cassette was inserted 3' to exon 1. The selection cassette was removed in ES cells by transient Cre expression prior to the production of chimeric mice. leaving a single loxP site 3' to exon 1 in the final allele. (J:72241)