U1 is a small nuclear RNA (snRNA) component of the spliceosome (involved in pre-mRNA splicing). Its 5' end forms complementary base pairs with the 5' splice junction, thus defining the 5' donor site of an intron. There are significant differences in sequence and secondary structure between metazoan and yeast U1 snRNAs, the latter being much longer (568 nucleotides as compared to 164 nucleotides in human). Nevertheless, secondary structure predictions suggest that all U1 snRNAs share a 'common core' consisting of helices I, II, the proximal region of III, and IV.