Cajal bodies are specialized and dynamic compartments in the nucleus that are 
involved in the biogenesis of small nuclear ribonucleoproteins (snRNPs). Because 
of the dynamic and varied roles of Cajal bodies, it is of great interest to 
identify the components of Cajal bodies to better understand their functions. We 
performed a genome-wide screen to identify proteins that colocalize with coilin, 
the marker protein of Cajal bodies. In this study, we identified and 
characterized Fam118B as a newly discovered component of Cajal bodies. Fam118B 
is widely expressed in a variety of cell lines derived from various origins. 
Overexpression of Fam118B changes the canonical morphology of Cajal bodies, 
whereas depletion of Fam118B disrupts the localization of components of Cajal 
bodies, including coilin, the survival of motor neuron protein (SMN) and the Sm 
protein D1 (SmD1, also known as SNRPD1). Moreover, depletion of Fam118B reduces 
splicing capacity and inhibits cell proliferation. In addition, Fam118B 
associates with coilin and SMN proteins. Fam118B depletion reduces symmetric 
dimethylarginine modification of SmD1, which in turn diminishes the binding of 
SMN to this Sm protein. Taken together, these data indicate that Fam118B, by 
regulating SmD1 symmetric dimethylarginine modification, plays an important role 
in Cajal body formation, snRNP biogenesis and cell viability.