The aim of this study was to investigate whether selenium-containing methionine (NASeLM: N-acetyl-selenomethionine), in contrast to non-selenium-containing methionine (NaLM: Na-acetyl-L-methionine), has an anti-proliferative and growth-inhibiting effect on two different cancer cells with high radical activity, such as Jurkat cells and MTC-SK, as well as increased caspase 3 activity, in a dose-dependent manner. This evidence concerns the gene CASP3 and cancer.