Cajal body-like structures called "gems" (gemini satellites of Cajal body) have recently been shown to be paired frequently with Cajal bodies. Sometimes the two structures overlap, and both are similarly affected by cell growth temperature and transcription inhibitors.

Gems contain the SMN (survival of motor neurons) protein, encoded by the gene responsible for a severe inherited form of human muscular wasting disease, spinal muscular atrophy. The SMN protein interacts with the Sm class of snRNP proteins and (through a separate binding site) with a cellular protein called SIP1. The SMN-SIP1 complex plays an essential role in cytoplasmic snRNP biogenesis.

Defects in spliceosomal snRNP assembly may thus be involved in spinal muscular atrophy, and an intranuclear snRNP trafficking pathway may involve interactions between gems and Cajal bodies.

HeLa cells stained with MANSMA I*, a monoclonal antibody to SMN. *Young, P.J., Le, T.T., thi Man, N., Burghes, A.H. and Morris, G.E. (2000) Exp. Cell Res. 256:365-374. image provide by Dr. Judith Sleeman