Richter's syndrome (RS), also known as Richter's transformation, is a transformation which occurs in about 5-10% of B cell chronic lymphocytic leukemia (CLL) and hairy cell leukemia into a fast-growing diffuse large B cell lymphoma, a variety of non-Hodgkin lymphoma which is refractory to treatment and carries a bad prognosis. There is also a less common variant in which the CLL changes into a Hodgkin's lymphoma. Rarely, transformations to a form of myeloid leukemia have been observed. These extraordinarily rare transformations carry a very poor prognosis. Richter's transformation affects about 5% of CLL patients at some point during their lives.