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Infection by the human immunodeficiency virus (HIV) is initiated by the binding of its extracellular envelope glycoprotein, gp120, to the CD4 antigen on target cells. To map the residues of the HIV-1 glycoprotein that are critical for binding and to analyse the effects of binding on viral infectivity, we created 15 mutations in a region of gp120 that is important for binding to CD4 (refs 4,5). We find that substitution of a single amino acid (tryptophan at position 432) can abrogate CD4 binding and that virus carrying this mutation is non-infectious. By contrast, other amino-acid changes in the same region do not affect CD4 binding but restrict viral tropism: virions containing isoleucine substitutions at position 425 lose their ability to infect a monocyte cell line (U937 cells) but can still infect T-lymphocyte cell lines (CEM, SUP-T1) and activated human peripheral blood lymphocytes. These results indicate that cellular tropism of HIV can be influenced by a single amino-acid change in gp120.

Xeroderma pigmentosum variant (XP-V) cells are deficient in their ability to synthesize intact daughter DNA strands after UV irradiation. This deficiency results from mutations in the gene encoding DNA polymerase , which is required for effecting translesion synthesis (TLS) past UV photoproducts. We have developed a simple cellular procedure to identify XP-V cell strains, and have subsequently analyzed the mutations in 21 patients with XP-V. The 16 mutations that we have identified fall into three categories. Many of them result in severe truncations of the protein and are effectively null alleles. However, we have also identified five missense mutations located in the conserved catalytic domain of the protein. Extracts of cells falling into these two categories are defective in the ability to carry out TLS past sites of DNA damage. Three mutations cause truncations at the C terminus such that the catalytic domains are intact, and extracts from these cells are able to carry out TLS. From our previous work, however, we anticipate that protein in these cells will not be localized in the nucleus nor will it be relocalized into replication foci during DNA replication. The spectrum of both missense and truncating mutations is markedly skewed toward the N-terminal half of the protein. Two of the missense mutations are predicted to affect the interaction with DNA, the others are likely to disrupt the threedimensional structure of the protein. There is a wide variability in clinical features among patients, which is not obviously related to the site or type of mutation. X eroderma pigmentosum (XP) is a rare autosomal recessive disorder characterized by extreme sensitivity of the skin to sunlight-induced changes (1). In particular, affected individuals have a 1,000-fold increase in the incidence of sunlight-induced skin cancers. Genetically, XP is complex and in the majority of patients the clinical features result from a defect in one of seven genes, XPA-G, controlling nucleotide excision repair (NER). The products of these XP genes are involved in one or other of the steps in the NER process (2, 3). Approximately 20% of patients with XP are so-called XP variants (XP-V), which are defined by their normal levels of NER. Cultured fibroblasts from XP-V donors are only mildly sensitive to the killing effects of UV light, in contrast to the NER-defective XP cells, which have very marked hypersensitivity. Both NER-defective XP and XP-V cells are hypersensitive to the mutagenic effects of UV irradiation (4-6).The defect in XP-V cells is a reduced ability to replicate DNA after irradiation (also referred to as postreplication repair). XP-V cells are unable to synthesize intact daughter DNA strands on UV-irradiated templates (7). In vitro studies showed that this deficiency resulted from an inability to carry out translesion synthesis (TLS), the synthesis of DNA directly past damaged sites (8). These and other in vitro studies led to the cloning of the gene deficient in patients with XP-V (9, 10), and the demonstration that the ...

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