A PGK-hygromycin resistance cassette replaced exon 1 of the wild-type Scx allele in heterozygous ES cells (Gt(ROSA)26Sor+/- Scxtm1Eno-+/-). Injection of lacZ-marked Scxtm1Eno/Scxtm2Eno ES cells into wild-type blastocysts resulted in chimeric embryos. (J:55991)