Neonatal Diabetes Mellitus (NDM) is a rare disease that may be transient or permanent. KCNJ11 gene mutations represent the most common cause of Permanent NDM but are only rarely implicated in Transient NDM – the motive behind this phenomenon is still unclear. Genetic diagnosis of NDM patients affects management strategy choices, as exemplified by the particularly successful use of exclusive sulfonylurea treatment on KCNJ11-related NDM. We present 2 case reports of patients that, despite having the same presenting condition and being later found to have the same mutated gene, had significantly different outcomes. While one illustrates the importance of genetic testing on therapeutic choices, the other shows the test’s limitation when it comes to predicting the patients’ final outcome – showing that even the most prevalent cause of permanent NDM can cause transient forms of the condition. For more information click here.