Conversion of pyruvate kinase (PK) isozymes from M2- to R-PK has been observed during erythroid cell maturation. To understand this mechanism, we analyzed the PK gene of a R-PK deficient patient, in whose erythrocytes the M2-PK was persistently expressed. A point mutation, 1102 GTC-->TTC was identified in the R-PK cDNA, and it caused a single amino acid substitution from 368Val-->Phe. The residue is very close to the 372nd Gln, the putative binding site of the monovalent cation (K+). The impaired K+ binding would cause the decreased affinity for phosphoenolpyruvate, consequently the variant PK may be extremely unstable. Although the proband's other PK allele did not have any structural change, the R-PK mRNA level in reticulocytes was decreased. These findings suggested that both the structural mutation near the active site and the decreased mRNA level of the R-PK were responsible for the disorder.