Normal adult with absent HEX A: immunoreactive HEX A is present.

Fibroblasts from a normal adult with absent hexosaminidase A (HEX A) activity were demonstrated to possess immunoreactive HEX A as measured by GM2 beta-D-N-acetylgalactosaminidase activity which precipitated with specific anti-HEX A antibodies. Possible explanations for the molecular defect are presented.