Other universal genes previously shown to be increased during the expansion of the β-cell during obesity or pregnancy such as Birc5, Igf1r and Rasgrp1 also failed to increase in PPARγ deficient ob/ob islets (POKO vs. WT) further suggesting an inability of POKO islets to proliferate at a young age. Here, PPARG is linked to obesity due to melanocortin 4 receptor deficiency.