Problem: A 20--year -old Asian American obese male is newly diagnosed with diabetes mellitus. His father (now age 53) was diagnosed with diabetes when he was 44 years old during a routine physical exam. His only sibling, his sister (now age 25) was diagnosed with impaired fasting glucose when she was 19-years old. His father achieved good glycemic control using two oral agents for several years but last year he required insulin therapy to achieve his glycemic goals. His sister began exercising and lost more than 40 lbs after learning her blood glucose was abnormal. Her most recent fasting blood glucose reading was 93 mg/dL (5.2 mmol/L) and her A1C was 5.4% (0.054; 36 mmol/mol Hb). What is the most likely etiology of their diabetes mellitus?