To understand more comprehensively the tumor responses that follow the activation of canonical BMP4 signalling through SMAD4, and non-canonical BMP4 signalling in the absence of SMAD4, we have utilized preclinical models of spontaneous breast cancer metastasis to investigate the impact of SMAD4 on the ability of BMP4 to regulate tumor growth and metastasis. The gene discussed is BMP4; the disease is breast cancer.