Size analysis of PCR products from genomic DNA. Primer 1 (sense) was chosen on intron 4, primer 2 (antisense) on intron 5, primer 3 (sense) on intron 5 and primer 4 (antisense) on intron 7 (for positions, see Fig 3; for compositions, see Table 1). Primers 2 and 3 hybridize with DNA sequences within the deletions of patient 1 and patient 2, respectively (Fig 3). Therefore, PCR (a), with primers 3 and 4, only leads to product formation with DNA from patient 1 (not with DNA from a healthy control donor because these primers then hybridize too far apart to lead to product formation under the PCR conditions used). PCR (b), with primers 2 and 3, only leads to product formation with DNA from a healthy control donor. PCR (c), with primers 1 and 2, only leads to product formation with DNA from patient 2. The figure shows the results of PCR (a), (b), and (c) obtained with DNA from a healthy control donor, patient 1 (the eldest brother), patient 2 (the youngest brother), the mother of the patients (3), and the maternal grandmother of the patients (4). The four lanes marked M contain size markers of 100 bp.