Mutations in the gene encoding the phosphoinositide phosphatase myotubularin 1 
protein (MTM1) are usually associated with severe neonatal X-linked myotubular 
myopathy (XLMTM). However, mutations in MTM1 have also been recognized as the 
underlying cause of "atypical" forms of XLMTM in newborn boys, female infants, 
female manifesting carriers and adult men. We reviewed systematically the 
biopsies of a cohort of patients with an unclassified form of centronuclear 
myopathy (CNM) and identified four patients presenting a peculiar histological 
alteration in some muscle fibers that resembled a necklace ("necklace fibers"). 
We analyzed further the clinical and morphological features and performed a 
screening of the genes involved in CNM. Muscle biopsies in all four patients 
demonstrated 4-20% of fibers with internalized nuclei aligned in a basophilic 
ring (necklace) at 3 microm beneath the sarcolemma. Ultrastructurally, such 
necklaces consisted of myofibrils of smaller diameter, in oblique orientation, 
surrounded by mitochondria, sarcoplasmic reticulum and glycogen granules. In the 
four patients (three women and one man), myopathy developed in early childhood 
but was slowly progressive. All had mutations in the MTM1 gene. Two mutations 
have previously been reported (p.E404K and p.R241Q), while two are novel; a 
c.205_206delinsAACT frameshift change in exon 4 and a c.1234A>G mutation in exon 
11 leading to an abnormal splicing and the deletion of nine amino acids in the 
catalytic domain of MTM1. Necklace fibers were seen neither in DNM2- or 
BIN1-related CNM nor in males with classical XLMTM. The presence of necklace 
fibers is useful as a marker to direct genetic analysis to MTM1 in CNM.