55X 55x S-{1-[5-(6-amino-9H-purin-9-yl)-4-hydroxy-3-(phosphonooxy)tetrahydrofuran-2-yl]-3,7-dihydroxy-6,6-dimethyl-3-oxido-8,12 -dioxo-2,4-dioxa-9,13-diaza-3lambda~5~-phosphapentadecan-15-yl} (2S,6R)-6-[(8S,9S,10R,13R,14S,17R)-10,13-dimethyl-3-oxo-2,3,6,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclopenta [a]phenanthren-17-yl]-2-methylheptanethioate (non-preferred name) 55X