A locus on chromosome 15q21-23 for a dominantly inherited nemaline myopathy with core-like lesions is reported in two unrelated families evaluated at University Medical Center, Nijmegen, The Netherlands. TPM1 considered the strongest candidate gene had no identified disease-associated mutations. Patients have muscle weakness in neck flexors and proximal limb muscles without involvement of facial, respiratory, foot dorsiflexors or cardiac muscles. Muscle weakness is very slowly progressive and is manifested by the inability to perform fast movements such as running, and failure to break a fall when stumbling. Quadriceps biopsies showed type 1 fiber predominance with granular appearance suggestive of nemaline rods with Gomori trichrome stain. Zones devoid of ATPase and oxidative staining resembled central cores (pseudocores). Ultrastructural fiber examinations showed nemaline rods. 
COMMENT. A novel nemaline core-like myopathy is described in two unrelated families with an uncharacterized genotype. Muscle weakness is manifested by slowness of movement and inability to break a fall.