PMP22 and hereditary neuropathy with liability to pressure palsies: Abnormal PMP22 in HNPP paradoxically leads to focal thickened myelin,[7] which is characterized by sausage-like expansions, referred to as tomacula.[8] With the widespread clinical use of genetic testing and the invasive nature of nerve biopsy, the role of nerve biopsy in the diagnosis of HNPP is limited, and peripheral nerve biopsy is only considered when there is high clinical suspicion of HNPP without positive genetic test results.