- Characterization of V3 sequence heterogeneity in subtype C human immunodeficiency virus type 1 isolates from Malawi: underrepresentation of X4 variants. Ping, L.H., Nelson, J.A., Hoffman, I.F., Schock, J., Lamers, S.L., Goodman, M., Vernazza, P., Kazembe, P., Maida, M., Zimba, D., Goodenow, M.M., Eron, J.J., Fiscus, S.A., Cohen, M.S., Swanstrom, R. J. Virol. (1999)