The transgene comprises a mutant human genomic placental alkaline phosphatase gene joined to the enhancer/promoter from the human beta-actin gene. The mutation was generated by inserting 7 G:C base pairs downstream of a series of 4 G:C pairs near the start of the reading frame, causing a frameshift that prevents production of a functional enzyme. Loss of a single base pair from the 11-G:C tract restores the reading frame and results in reversion to the wild-type enzymatic phenotype. (J:86017)