The invention provides a method for determining whether a human immunodeficiency virus is likely to be more resistant to a viral entry inhibitor than a reference HIV. In certain aspects, the methods comprise comparing the length of one or more variable regions of an envelope protein of the HIV or a number of glycosylation sites on the envelope protein of the HIV to a length of one or more corresponding variable regions of an envelope protein of the reference HIV or a number of glycosylation sites on the envelope protein of the reference HIV, wherein the HIV is likely to be more resistant to the CD4 binding site entry inhibitor than the reference HIV when the HIV has longer variable regions than the reference HIV or the HIV has more glycosylation sites than the reference HIV.