The CLL cells from each of several venetoclax-resistant patients have mutations in BCL2, most commonly BCL2G101V, generated by a nonsynonymous mutation affecting a glycine to valine substitution at amino-acid-position 101 of BCL2. Venetoclax binds poorly to the mutant BCL2 encoded by BCL2G101V and thus is less able to induce CLL-cell apoptosis. This evidence concerns the gene BCL2 and B-cell chronic lymphocytic leukemia.