Isolated CI enzyme defect was found in skin fibroblast and muscle samples from three individuals.8,9 Our cumulative phenotypic analysis of nine new and two previously reported families suggested that the phenotype of defective NDUFA13 encompasses mostly infantile-onset and non-rapidly progressing neurological impairment on the background of severe-to-profound and moderate global and isolated developmental delay. This evidence concerns the gene NDUFB6 and Global developmental delay.