Reducing Body Myopathy is rare, and has a variable course.

The defining feature is the presence of "reducing bodies," which are small, roundish bodies that often are found near the nuclei in muscle fibers.  They can be visualized with trichrome.  It's unclear what they are.

The genetics remain unclear.

References:

Katirji, B., Kaminsky, H.J., Preston, D.C., Ruff, R.L., and Shapiro, B.E. (Eds.) (2002). Neuromuscular Disorders in Clinical Practice. Butterworth Heinemann, Boston. (ISBN:0750671696)  Pp. 1114-1127.