 A defining characteristic of MCL is mutation and overexpression of cyclin D1, a cell cycle gene, that contributes to the abnormal proliferation of the malignant cells.MCL cells may also be resistant to drug-induced apoptosis, making them harder to cure with chemotherapy or radiation.Cells affected by MCL proliferate in a "nodular" or "diffuse" pattern with two main cytologic variants, "typical" or "blastic".