What effect will a frameshift at the start of the second exon of an HLA-B gene most likely cause?

A frameshift mutation occurs when there is an insertion or deletion of nucleotides in a DNA sequence that alters the reading frame of the gene. In the context of the HLA-B gene, a frameshift at the start of the second exon would disrupt the normal sequence of codons that follow that point. As a result, this alteration would likely cause the translation machinery to read the mRNA incorrectly, leading to the production of an entirely different sequence of amino acids from that point onward.

This alteration typically continues until a stop codon is encountered, which is likely to happen prematurely due to the shift in the reading frame. A premature stop codon results in a truncated protein, which is generally nonfunctional. In the case of HLA-B, this would negatively impact its role in immune function, specifically in presenting antigens to T cells, which is critical for immune responses.

The correct answer highlights this mechanism, illustrating that a frameshift mutation can have drastic effects on the resulting protein product, potentially leading to loss of function or altered function due to protein truncation.