Detection of a homoplasmic polymorphism in patient no. 1 and her two daughters. PCR-amplified mtDNA segment no. 9 (nt 8282-9262) was treated with Taq I. According to the Cambridge sequence, Taq I has a restriction site at nt 8957, which divides the segment of 981 bp into two fragments of 676 and 305 bp. In patient no. 1 (lane P1) and her two daughters (D1 and D2), complete (homoplasmic) loss of the restriction site left the PCR product intact after Taq I digestion, whereas the expected cleavage occurred in four other patients (C1 through 4). Lane 4, DNA size marker (pGEM).