Heterozygous versus homozygous phenotype caused by the same MC4R mutation: novel mutation affecting a large consanguineous kindred.

MC4R mutation homozygotes exhibited morbid early-onset obesity, while heterozygotes had a significantly milder overweight phenotype. Whereas obesity due to MC4R mutations is evident as of early age - most notably in homozygotes, the metabolic consequences emerge only later in life.