The 3' untranslated region (UTR) was modified to include a floxed neo cassette, an internal ribosomal entry site (IRES) and a non-signaling form of the human nerve growth factor (NGFR) gene. This results in IRES mediated translation of NGFR when the endogenous locus is activated. Cre-mediated recombination removed the neo cassette. (J:187909)