Acyclic nucleoside phosphonate analogues, markedly enhance natural killer activity in mice : V. Del Gobbo∗, C.F. Perno∗, E. Balestra∗, F. Sesa∗, J. Balzarini§, E. De Clercq§, J. Jindrich÷, A. Holy÷, N. Villani∗ and R. Calio∗ ∗Department of Experimental Medicine, II University of Rome “Tor Vergata”, Italy, §Rega Institute for Medical Research, Katholieke Universiteit Leuven, Belgium and ÷Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia