The thymidine analog 3'-azidothymidine (AZT) blocks replication of human immunodeficiency virus (HIV). AZT is converted in cells to the 5'-triphosphate nucleotide derivative and then incorporatioed into cDNA copies of the infecting HIV RNA by the HIV's own reverse transcriptase. The cDNA terminates prematurely at the point of the inserted analog. The most likely reason that AZT is not equally inhibitory toward replication of human DNA is that the 5'-triphosphate nucleotide derivative is ?