The rudimentary human β-cell mitogenic signaling road map. The rudimentary human β-cell mitogenic signaling road map. Contrast this figure to Fig. 1. The gray lines are molecules and pathways that are known to exist in rodents but are unstudied in human β-cells. The human β-cell signaling road map is underdeveloped: Although the human G1/S molecules are reasonably well-characterized in the human β-cell (66), only a handful of growth factor receptors (e.g., IR, IGF-1R, and c-Met) and signaling molecules (IRS-1, IRS-2, PI3K, Akt, PKCζ, and GSK3β) are known to be present in human β-cell, and the majority of the proteins involved in signaling have not been studied. Rohit N. Kulkarni et al. Diabetes 2012;61:2205-2213 ©2012 by American Diabetes Association