SummaryWe have confirmed the localization of human acid alpha‐glucosidase (GAA) to 17q21 → q25 and of adenosine deaminase (ADA) to 20q13 → 20qter by examination of hybrid clones derived from a fusion between a human cell line carrying a 17/20 balanced translocation (17pter → 17q25:: 20q13 → 20qter; 20pter → 20q13:: 17g25 → 17gter) and a mouse line deficient in thymidine kinase. These hybrids were constantly maintained in HAT selective media in order to select for the presence of the human thymidine kinase gene on the intact chromosome 17 (17q21 → 22) or the 17/20 (17pter → 17q25:: 20q13 → 20qter) translocation chromosome. We detected human GAA by rocket immunoelectrophoresis, using a heterologous antibody raised against human acid alpha‐glucosidase. A clone which contained the 17/20 translocation and no intact chromosome 17 was still positive for GAA. This finding confirms the exclusion ofGAAfrom 17q25 ‐0 17qter reported by Nickelet al. (1982). Combined with earlier results (Weilet al. 1979),GAAcan be assigned to 17q21 → 17q25. A clone which contained only the 17/20 translocation chromosome and no intact chromosome 20 contained ADA. This confirms the previous localization ofADAto 20q13.2 → qter by gene dosage