The first five-helix bundle is a conserved cyclin box, a region of about 100 amino acid residues on all cyclins, which is needed for Cdk binding and activation. The second five-helix bundle is composed of the same arrangement of helices, but the primary sequence of the two subdomains is distinct.[6] All three D-type cyclins (D1, D2, D3) have the same alpha 1 helix hydrophobic patch. However, it is composed of different amino acid residues as the same patch in cyclins E, A, and B.[6]