Although the exact cause of CLL is unknown, CLL occurs when there is damage to the genes of developing lymphocytes in the bone marrow. B lymphocytes (B cells) are the type of white blood cell that becomes cancerous in CLL. This damage changes the normal lifespan of the lymphocytes and they do not die as planned. The increasing number of CLL cells in the blood and bone marrow crowd out healthy blood cells. This can disrupt the ability of the healthy blood cells to carry out their regular functions.