The human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins medi
ate virus entry into target cells by binding receptors of the cell membrane
and fusing viral and cellular structures. In particular, recent crystallog
raphic studies have clarified the complex role of the glycoprotein gp120 in
the early phase of the infection. In this context the inter- and intra-hos
t variability of the HIV-1 gp120 poses a major problem for the development
of effective methods of immunization against this virus. In the present rep
ort, the relevant aspects emerging from the study of HIV-1 variability are
addressed and several methodological approaches to evaluate HIV-1 diversity
discussed.