We found that caspase-9 was activated after treatment with [Bi-213]anti-CD20 in radio-/chemosensitive as well as in radio-/chemoresistant CD20-positive NHL cells, demonstrating that the mitochondrial pathway is involved in apoptosis induction and deficient activation of caspase-9 and of mitochondria is reversed in radio-/chemoresistant CD20-positive NHL cells after [Bi-213]anti-CD20 treatment. This evidence concerns the gene CASP9 and non-Hodgkin lymphoma.