Component of the spliceosomal U1 snRNP, which is essential for recognition of the pre-mRNA 5' splice-site and the subsequent assembly of the spliceosome. snrnp70 binds to the loop I region of U1-snRNA. Component of the U1 snRNP. The U1 snRNP is composed of the U1 snRNA and the 7 core Sm proteins snrpb, snrpd1, snrpd2, snrpd3, snrpe, snrpf and snrpg that assemble in a heptameric protein ring on the Sm site of the small nuclear RNA to form the core snRNP, and at least three U1 snRNP-specific proteins snrnp70/U1-70K, snrpa/U1-A and snrpc/U1-C. The RRM domain mediates interaction with U1 RNA.