GCG and neonatal diabetes mellitus: On follow-up, serial oral glucose tolerance tests (OGTT), HbA1c assessments, and glucagon stimulation tests to date demonstrated preserved β-cell function, with the exception of hyperglycemia in 30-60 min of the extended OGTT in one patient.<h4>Conclusions</h4>These two cases represent the first report of transient NDM due to HNF1B deletion.