Expression of Tec and Btk in human B cells. After immunoprecipitation with rabbit antisera to Tec or Btk, Western blots were probed with goat antisera against these proteins. (A) Tec was expressed in all B-cell lines studied representative of different stages of maturation, including two EBV-transformed lymphoblastoid cell lines derived from patients with XLA and lacking Btk expression. The myeloid cell line K562 and the T-lymphoid cell line Jurkat were used as a positive and negative control, respectively. (B) Tec and Btk were also expressed in tonsillar lymphoid cells, but not in thymocytes. Molecular mass markers (in kD) are indicated. The intense band of approximately 50 kD corresponds to the Ig heavy chain of the antibody used for immunoprecipitation.