33407-74-8 99211-98-0 Cyclobuta(1,2-d:4,3-d')dipyrimidine-2,4,5,7(3H,6H)-tetrone, 1,8-bis(2-deoxy-beta-D-erythro-pentofuranosyl)hexahydro-4a,4b-dimethyl-, (4aalpha,4balpha,8aalpha,8balpha)- Cyclobuta-dithymidine photoproducts Cyclobutyl dithymidine dimers Cyclobutyldithymidine dimers