Past exam of the biochemistry and Molecular Biology course of the University of Cambridge / 2025 / Paper 2 / Section C / 14 / i

The lower region can contain both the genuine TCF-short product and a truncated product derived from the exon-5-containing TCF-long transcript. A repeat frameshift mutation creates an early stop codon, producing a long-derived product close in size to TCF-short. The N-terminal antibody recognizes their shared retained region. The two nearby bands need not represent two copies of the normal short splice product. In patient 1's healthy sample, one mutated allele permits a truncated product alongside intact long and short products; in the tumor, mutation of both alleles explains loss of the intact long band while short-sized products remain. Test the assignment with an exon-5/C-terminal-specific reagent or peptide identification; phosphorylation or other modifications could also create doublets, so mobility alone is not definitive.