While the myeloid Src family kinase HCK is known to regulate immune evasion, here we show that HCK promotes growth of an aggressively invasive mammary tumor through activation of macrophage motility and invasive capacity.<h4>Method</h4>We used the Py8119 mouse mammary tumor model to investigate the role of Hck activity in tumor growth through therapeutic inhibition and genetic modification. The gene discussed is HCK; the disease is breast cancer.