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: Biochemistry
Amber codon refers to: 
Select and explain. 
A. Mutant codon, B. Stop codon, C. Initiating codon, D. Codon for more than one ammo acids
B: Stop codon
Answer: B: Stop codon 
Explanation: UAG the first codon to be deciphered was termed as amber codon. Mutants that are defective owing to the presence of an abnormal amber codon are called amber mutants. The other two stop codons are UGA and UAA. UGA is called the opal codon and UAA is called the ochre codon. Mutants that are defective because they contain abnormal opal or ochre codons are called opal and ochre mutants. Stop codons are called nonsense codons because they designate no amino acids. The initiating codon in mRNA is AUG and rarely GUG. Ref: Introduction to Genetic Analysis (Loose-Leaf) By Anthony J.F. Griffiths, page 330.