Two 2 -deoxynucleoside 5 -α-methylenephosphonyl-β,γ-diphosphates were synthesized. They were incorporated into the DNA chain by DNA polymerase α from human placenta. Meanwhile, they were not recognized by DNA polymerases ε and β of the same origin as well as by reverse transcriptases from human immunodeficiency virus and avian myeloblastosis virus.