We hypothesized that Spy1 cooperates with loss of p53 to increase susceptibility to tumour initiation due to changes in susceptible mammary stem cell populations during development and drives the formation of more aggressive stem like tumours.<h4>Methods</h4>Using a transgenic mouse model driving expression of Spy1 within the mammary gland, mammary development and stemness were assessed. The gene discussed is TP53; the disease is neoplasm.