We present comparisons of the clinical pictures in a series of 60 patients with 
nemaline myopathy in whom mutations had been identified in the genes for nebulin 
or skeletal muscle alpha-actin. In the patients with nebulin mutations, the 
typical form of nemaline myopathy predominated, while severe, mild or 
intermediate forms were less frequent. Autosomal recessive inheritance had been 
verified or appeared likely in all nebulin cases. In the patients with actin 
mutations, the severe form of nemaline myopathy was the most common, but some 
had the mild or typical form, and a few showed other associated features such as 
intranuclear rods or actin accumulation. Most cases were sporadic, but in 
addition there were instances of both autosomal dominant and autosomal recessive 
inheritance, while two families showed mosaicism for dominant mutations. 
Although no specific phenotype was found to be associated with mutations in 
either gene, clinical and histological features together with pedigree data may 
be used in guiding mutation detection. Finding the causative mutation(s) 
determines the mode of inheritance and permits prenatal diagnosis if requested, 
but will not as such permit prognostication.