We describe a young woman who presented with a progressive myopathy since the age of 9. Spectrophotometric analysis of the respiratory chain in muscle tissue revealed combined and profound complex I, III, II + III, and IV deficiency ranging from 60% to 95% associated with morphological and histochemical abnormalities of the muscle. An exhaustive screening of mitochondrial transfer and ribosomal RNAs showed a novel G > A substitution at nucleotide position 3090 which was detected only in urine sediment and muscle of the patient and was not found in her mother's blood cells and urine sample. We suggest that this novel de novo mutation in the 16S ribosomal RNA, a nucleotide which is highly conserved in different species, would impair mitochondrial protein synthesis and would cause a severe myopathy. (c) 2007 Elsevier Inc. All rights reserved.