IGF1 and breast cancer: The latter interpretation would be consistent with findings from some (Hankinson et al, 1998; Toniolo et al, 2000), but not all (Kaaks et al, 2002), prospective studies showing that high serum levels in adulthood of insulin-like growth factor-I (IGF-I), a hormone that promotes somatic growth, are associated with an increased risk and that, similar to the effect of birth weight, the IGF-I association with breast cancer seems to be particularly stronger at premenopausal ages.