IGF1R and breast cancer: In addition, IGF-1R is highly homologous to another tyrosine kinase receptor, namely the insulin receptor (IR).54 Of note, numerous in vitro and in vivo studies have demonstrated the correlation between IR overexpression and malignancy in several cancer models, including breast cancer.55 The two structurally different isoforms of IR, IR-α and IR-β, are overexpressed in many cancers.56 IR-α is the main isoform that plays an important role in cancer cell stemness, in tumour progression and in resistance to IGF-1R-targeted therapies (for review, see ref. 56).