This domain family is found in eukaryotes, and is typically between 212 and 238 amino acids in length. The family is found in association with pfam01805. There are two completely conserved residues (W and H) that may be functionally important. PRP21 is required for assembly of the prespliceosome and it interacts with U2 snRNP and/or pre-mRNA in the prespliceosome. This family also contains proteins similar to PRP21, such as the mammalian SF3a. SF3a also interacts with U2 snRNP from the prespliceosome, converting it to its active form.