The Cajal body, originally identified over 100 years ago as a nucleolar 
accessory body in neurons, has come to be identified with nucleoplasmic 
structures, often quite tiny, that contain coiled threads of the marker protein, 
coilin. The interaction of coilin with other proteins appears to increase the 
efficiency of several nuclear processes by concentrating their components in the 
Cajal body. The best-known of these processes is the modification and assembly 
of U snRNPs, some of which eventually form the RNA splicing machinery, or 
spliceosome. Over the last 10 years, research into the function of Cajal bodies 
has been greatly stimulated by the discovery that SMN, the protein deficient in 
the inherited neuromuscular disease, spinal muscular atrophy, is a Cajal body 
component and has an essential role in the assembly of spliceosomal U snRNPs in 
the cytoplasm and their delivery to the Cajal body in the nucleus.