Non-receptor tyrosine kinase Chk has been implicated in hematopoietic development. To study the function of Chk in vivo, we have generated chk-deficient mice using gene targeting. Overall development of mice homozygous for this mutation was apparently normal. Blood counts, FACS analysis of hematopoietic cell populations, CFU-C and CAFC assays showed no significant difference between wild type and mutant animals. Thus, the dispensability of Chk for mouse development and hematopoiesis suggests that its function may be redundant in vivo, and most likely be compensated by activity of a closely related protein tyrosine kinase Csk.