The molecular basis of triosephosphate isomerase (TPI) deficiency was studied in 3 patients from three separate families. In all 3 patients, genomic DNA directly sequenced after amplification by the polymerase chain reaction exhibited the point mutation TPI315C amino acid 104 Glu-->Asp. Although other mutations known to cause TPI deficiency have been restricted to single families, the amino acid 104 defect has now been described in nine apparently unrelated families throughout the world and is clearly the most frequently occurring form of the disorder. The basis of the repetitive occurrence of this mutation remains unexplained.