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Deoxyadenosine diphosphate is a derivative of the common nucleic acid ATP, or adenosine triphosphate, in which the -OH (hydroxyl) group on the 2' carbon on the nucleotide's pentose has been removed (hence the deoxy- part of the name). Additionally, the diphosphate of the name indicates that one of the phosphoryl groups of ATP has been removed, most likely by hydrolysis.
Deoxyadenosine diphosphate would be abbreviated dADP.