In 1969, significant C-peptide immunoreactivity is detectable in this patient’s plasma, indicating that endogenous insulin secretion is present. As a result, diet therapy is effective. Over time, C-peptide concentrations fall and become undetectable, and this loss of endogenous insulin secretion corresponds with the need to use exogenous insulin injections to control blood glucose. Reproduced with permission from the New England Journal of Medicine (3).