Whereas most people with diabetes mellitus have type 1 (T1DM) or type 2 (T2DM) diabetes, there are a number of other inherited forms of diabetes and insulin resistance syndromes, which represent up to about 5% of all cases of diabetes. Monogenic forms of pancreatic β-cell dysfunction include maturity-onset diabetes of the young (MODY) and neonatal diabetes (NDM), with MODY being the most common form of inherited diabetes. The long-term response to sulfonylurea drugs in MODY caused by HNF1A and HNF4A mutations and NDM caused by KCNJ11 and ABCC8 mutations is an excellent example of pharmacogenetics.