The recently published WHO Diabetes Classification Report 2019 lists a number of "other specific types" [of diabetes], including monogenic diabetes and what was once called "secondary diabetes."
Monogenic diabetes, as the name suggests, is the result of a single gene and not the contribution of several genes and environmental factors, as seen in type 1 and type 2 diabetes. Monogenic diabetes is much less common, accounting for 1.5–2% of all cases, although this may well be underestimated. It is often misdiagnosed as type 1 or type 2 diabetes.