In 1980 the clinical syndrome of X-linked hypogammaglobulinemia and isolated growth hormone deficiency (XLA/GHD) was described. XLA/GHD patients have reduced serum levels of Ig and normal cell-mediated immunity, and thus resemble patients with Bruton's X-linked agammaglobulinemia (XLA). However, XLA/GHD patients also have isolated GHD. Mutations and deletions in the Bruton's tyrosine kinase gene (BTK) are responsible for Bruton's XLA. We investigated BTK gene expression in an XLA/GHD patient from the family originally described by Northern analysis, cDNA sequencing, and Western analysis of protein production using mAb to BTK. BTK mRNA was normal in size and abundance, and the mRNA sequence was normal over the coding region, except for a single silent mutation. BTK protein was present in normal amounts in PBMC of this patient. Thus, at the molecular level, XLA/GHD is a different disease entity from Bruton's XLA. These results suggest that undescribed genes critical for B cell development and growth hormone production exist on the X chromosome.