The entire coding sequence of a Japanese class 1 variant (G6PD Tokyo) was amplified by the polymerase chain reaction from genomic DNA. Nucleotide analysis by a direct sequencing technique revealed a unique nucleotide substitution (1246 G to A) in exon 10, which predicts a Glu to Lys substitution at the 416th amino acid. This is another member of a conspicuous mutation cluster surrounding the putative NADP-binding domain.