Fingerprint inclusions. Ultrastructural demonstration of muscle fiber type specificity.

Fingerprint inclusions were identified in the skeletal muscle of a child with severe hypotonia. The mean Z-line and M-line widths from each of 19 muscle fibers containing the fingerprint inclusions were calculated. The mean Z-line widths were 85-99 nm, and the mean M-line widths were 75-101 nm. According to out present system of fiber typing, all of these fibers would be classified as Type I. The possible pathogenesis and fiber type specificity of the fingerprint inclusion is discussed.