Though six IRS family members have been identified (IRS-1 to IRS-6) (White et al, 1985; Tobe et al, 1995; Lavan et al, 1997a, 1997b; Cai et al, 2003), IRS-1 and -2 are the predominant signalling molecules utilised by the IGF-IR to mediate IGF-I action in breast cancer cells (Jackson et al, 1998, 2001). Here, IRS1 is linked to breast cancer.