Abstract: The invention provides methods of screening a subject for a proliferative disease risk factor which comprises detecting the presence or absence of target cells. The presence of target cells in the subject indicates the subject is at increased risk of developing a proliferative disease or recurrence of a previously treated proliferative disease and defines a new patient subgroup susceptible for treatment with selected anti-proliferative compounds. Methods of screening compounds for the treatment of proliferative diseases based on the presence of target cells and the expression of certain biomarkers by said target cells are also disclosed, along with appropriate compounds and their use in methods of treating such diseases.