This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Gene for folate carrier protein is located on chromosome 
Select and explain. 
A. Chromosome 21, B. Chromosome 5, C. Chromosome 10, D. Chromosome 9
A: Chromosome 21
Answer: A: Chromosome 21 
Explanation: Folate cofactors are one-carbon donors essential for the biosynthesis of purines and thymidylate. Mammalian cells are devoidof folate biosynthesis and are therefore folate auxotrophs that take up folate vitamins primarily via the reduced folate carrier (RFC). The gene for RFC is located on chromosome 21.