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Two related studies were brought to our attention since the publication of our manuscript. Neither study referred in its title or abstract to methods for allele quantitation and were therefore unknown to us, but we would have cited them if we had known of them. One study describes the addition of a radiolabeled primer to the final cycle of PCR to distinguish between single base pair mutations1; the other describes the addition of radioactive dNTPs to the final cycle to distinguish mitochondrial DNA species2. Neither demonstrates the ability to quantitate varying allele ratios with precision.

1. Schrijver, I., Liu, W., Brenn, T., Furthmayr, H. & Francke, U. Cysteine substitutions in epidermal growth factor-like domains of fibrillin-1: distinct effects on biochemical and clinical phenotypes. Am. J. Hum. Genet. 65, 1007–1020 (1999).

2. Moraes, C.T., Ricci, E., Bonilla, E., DiMauro, S. & Schon, E.A. The mitochondrial tRNA(Leu(UUR)) mutation in mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS): genetic, biochemical, and morphological correlations in skeletal muscle. Am. J. Hum. Genet. 50, 934–949 (1992).