Whereas AT3‐OVA cells grew readily in Ptpn2fl/fl mice, tumour growth was markedly repressed in Lck‐Cre;Ptpn2fl/fl mice so that tumour progression was prevented in 5/13 mice and eradicated in 2/8 of the remaining mice after tumours had developed. The gene discussed is LCK; the disease is neoplasm.