T ranscription factor-7–like 2 (TCF7L2) is the most important type 2 diabetes susceptibility gene identified to date, with common intronic variants strongly associated with diabetes in all major racial groups. This ubiquitous transcription factor in the Wnt signaling pathway was not previously known to be involved in glucose homeostasis, so defining the underlying mechanism(s) will provide new insights into diabetes. In this issue of the JCI, Lyssenko and colleagues report on their human and isolated islet studies and suggest that the risk allele increases TCF7L2 expression in the pancreatic β cell, reducing insulin secretion and hence predisposing the individual to diabetes (see the related article beginning on page 2155).