- Efficiency of phosphorylation of the cyclobut-G (A-69992) enantiomers by HSV-1 thymidine kinase does not correlate with their anti-herpesvirus activity. Kohlbrenner, W.E., Carter, C.D., Fesik, S.W., Norbeck, D.W., Erickson, J. Biochem. Pharmacol. (1990)