The molecular safety of insulin analogues has received a great deal of attention over the last year. In particular, attention has been directed to the mitogenic properties of insulin analogues as compared with human insulin. Understanding the mechanisms implicated in mediating mitogenic effects of insulin is therefore of particular interest. In this review we detail the story of the rapid-acting insulin analogue known as X10, which was the first insulin analogue in clinical development, but ended up being discontinued at an early clinical development stage following findings of mammary tumours in female Sprague-Dawley rats. The molecular characteristics of insulin X10, along with its interaction at both the IGF-1 receptor and the insulin receptor, have provided us with important insights into mechanisms implicated in metabolic and mitogenic signalling of insulin analogues.