The U1 snRNP is a particle consisting of the U1 snRNA and Sm core plus unique snRNP polypeptides. The U1-specific polypeptides are 70K, A, and C. The Sm core polypeptides are B, B', D1, D2, D3, E, F, G. The Sm core is a heteroheptamer in the shape of a donut, containing either B or B', which are almost identical to each other.
