PLEC and Menkes disease: In contrast, the forced expression of any of the other skeletal muscle-specific plectin isoforms (P1, P1b, or P1f) showed no such effect, indicating that the striking sarcomere pathology in EBS-MD muscle is primarily due to a loss of function of isoform P1d, which, in partnership with desmin IFs, cross-links individual myofibrils to each other at the Z-disk level.