Human breast cancer xenografts only rarely invade and metastasize in nude mice, and have therefore only had limited use as a model for studying mechanisms involved in breast cancer spreading. However, recent reports describe differences not only between various cell lines but also between strains of immune-deficient mice in terms of providing a model for studies of the invasive and metastatic capability of human breast cancer xenografts. Genetic labelling of human cancer cells with the lacZ gene is described as a specific and highly sensitive method for identification of micrometastases in such a model.