Question: Arizona Subscriber Answer: 3-MCC is an uncommon, inherited disorder in which the body is unable to process certain proteins correctly because of a shortage of leucine. Infants with 3-MCC deficiency appear normal at birth but develop signs and symptoms in infancy or early childhood. Characteristic features of 3-MCC include feeding difficulties, recurrent episodes of vomiting or diarrhea, excessive tiredness, and weak muscle tone.