PGR and breast carcinoma: To explore this, we first assessed the growth of syngeneic tumours arising from ovalbumin (OVA)‐expressing AT‐3 (AT‐3‐OVA) mammary carcinoma cells implanted into the inguinal mammary fat pads of Ptpn2fl/fl versus Lck‐Cre;Ptpn2fl/fl C57BL/6 mice (Fig 1C); AT‐3 cells lack oestrogen receptor, progesterone receptor and ErbB2 expression and are a model of triple‐negative breast cancer (Stewart & Abrams, 2007; Mattarollo et al, 2011).