Embryonic development of the pancreas depends heavily on the Pdx1 transcription factor, which contributes to the differentiation and function of every cell type, including the insulin-secreting β-cells of the Islets of Langerhans. Expression of the gene encoding Pdx1 is controlled by four 5′-flanking enhancer-like domains designated Areas I, II, III, and IV. A variety of transcription factors bind to these areas with changes in identity and quantity occurring throughout development and after birth. Prior work revealed that Area II is necessary for development of pancreatic progenitor cell formation, leading Vanderbilt Basic Sciences investigators Roland Stein, Christopher Wright, and Mark Magnuson to investigate the role of Area IV. To accomplish their goal, they created a mouse bearing a Pdx1 gene lacking area IV. Mice homozygous for this gene exhibited no significant abnormalities, suggesting that the other regulatory areas could compensate for the Area IV deficit. However, heterozygous mice bearing one area IV knockout gene and one Pdx1-null gene exhibited a small but consistent reduction in the number of insulin-producing cells during embryogenesis. These mice appeared normal at birth and through the first several weeks of life, but at the time of weaning (>week 3), hyperglycemia appeared in the males. Further studies in these male mice demonstrated a reduction in β-cell area and proliferation by weeks 4-5. Consistently, a decrease in expression of β-cell-associated genes, and genes regulating endoplasmic reticulum function and the cell cycle was also evident. Because these abnormalities only occurred in mice expressing low levels of Pdx1 due to knockout of one allele, the investigators hypothesized that binding of Pdx1 itself to Area IV must occur at the time of weaning. Experimental data supported this idea. Weaning is associated with a marked change in diet that leads to differential expression of 1600 β-cell genes, over 500 of which are regulated by Pdx1. Together, the findings support a role for Pdx1 in the β-cell response to weaning. The work is published in the journal Diabetes [J.M. Spaeth, et al., (2017) Diabetes, published online July 13, DOI: 10.2337/db16-1516].