The human HEXB (hexosaminidase B) gene and human growth hormone polyA sequences followed by a loxP-flanked neomycin cassette are inserted into intron 1 of Gt(ROSA)26Sor placing HEXB expression under the control of the ROSA26 promoter. HEXB is modified by site-directed mutagenesis to harbor an arginine to glutamine mutation at residue 505 (R505Q). Cre-mediated recombination removed the floxed neo cassette. (J:94077)