Pathogenic mtDNA mutations causing mitochondrial myopathy: the need for muscle biopsy by Hardy, S, Blakely, E, Purvis, A, Rocha, M, Ahmed, S, Falkous, G, Poulton, J, Rose, M, O'Mahony, O, Bermingham, N, Dougan, C, Ng, Y, Horvath, R, Turnbull, D, Gorman, G, Taylor, R