= Solution
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. \b[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.
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