PAH gene mutation Analysis of the DNA of the Phenylalanine hydroxylase (PAH) gene in a patient with phenylketonuria revealed a mutation in the protein coding region whose predicted effect would be to replace the amino acid aspartic acid, with histidine. Nevertheless, no mutant protein could be found in the patient’s cells.How this could be as mutation is in the protein coding region,can anyone explain?

In terms of the way that the question is worded, I imagine the most likely answer would be that the amino acid substitution D > H disrupts the folding of the protein so that it is recognised as being aberrant and is rapidly degraded.