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How did the stop codon mutation in Lucy's ADA gene stop her ADA protein from working?

Adenosine deaminase or ADA gene is an enzyme which contribute in purine metabolism. It breaks the adenosine present in food and convert nucleic acids in tissues. It function in the maintenance and development of the immune system.

Stop codon signal the end of protein formation during translation and they do not code for an amino acid.

Stop codon will terminate the protein synthesis in ADA gene as it do not code for any amino acid and will cause nonsense mutation which will affect the immune system of the body.