The S5 and S6 transmembrane domains and the pore loop containing the selectivity filter of the gene were replaced with an IRES-PLAP-pA cassette. The resulting sequence produces a bicistronic transcript containing the 5' end of the endogenous gene followed by an independently translated human placental alkaline phosphatase gene. An endoplasmic reticulum retention signal encoded by the amino acid sequence KDEL and a stop codon were inserted in frame with the exon before the IRES-PLAP to prevent unwanted side effects from a possibly truncated product of the endogenous coding sequence. RT-PCR confirmed absence of transcript in mutants.