The '18' variant of human placental alkaline phosphatase is identical to the 'D-variant'.

Samples of the rare SD, FD and ID phenotypes of placental alkaline phosphatase were compared by starch gel electrophoresis and by measuring L-leucine inhibition with the numbered phenotypes of Donald and Robson. The 'D-variant' was found to be identical to 'variant 18'. 'Variant 17' was also found to be inhibited by L-leucine to the same extent as 'variant 18'. There was a statistically significant excess of male births with the variant 18 enzyme.