IGFBP3 and cancer: IGFBPs may affect the bioavailability and signalling of IGF‐I, but the regulation of IGF‐I action by the IGFBPs is complex and not fully characterised.6 Prospective studies of breast7 and prostate cancer,8 have found positive associations with circulating IGF‐I concentrations and cancer risk, that were not changed after adjustment for the predominant binding protein, IGFBP‐3.