We incubated Bcap-37 cells in culture medium containing 20 mM lactate and determined time-dependent lactate uptake. Cellular tropism may reflect changes in AbMole Trihexyphenidyl HCl target cell availability, leader peptide evolution may reflect adaptation from a low viral load, target cell rich environment to a high virus load, target cell limited environment. We have modeled the role of viral infectivity in very early and in steady-state infection. Infectivity may be most important during the virus ramp up phase when sufficiently activated target cells are limited. We show, however, that at viral set-point, the ability of a variant to achieve numerical superiority through high reproductive rates appears to be more important than its ability to compete for a limited number of individual target cells via an enhanced infectivity. But why might the position 12 signature be preferentially lost during chronic infection? Lowering envelope expression levels may be advantageous during chronic infection to escape anti-viral antibodies. Alternatively, different target cell populations may respond differently to changes in the signal peptide. The shift from CCR5 to CXCR4 tropism can potentially be explained by a shift in target cell populations as the virus expands into new niches. It is unclear if comparable cell type specificity in the position 12determined phenotype plays a role late in infection, and whether the transmission phenotype may be lost, or become neutral. Studies of additional HIV-1 envelope signatures, their temporal and spatial association with the position 12 signature, and their biological effects will provide a more complete understanding of the selection pressures faced by the virus during acute and chronic infection. According to the current view, CLL cells are highly dependent on microenvironmental interactions that provide proliferative and prosurvival stimuli to the malignant cells. CLL is a heterogeneous disease with a highly variable clinical course and a number of molecular prognostic markers have been identified to help determine that course. Among these are VLA-4 and CD38, two surface molecules that are believed not only to be mere markers of disease aggressiveness but also to play a role in CLL pathogenesis. VLA-4 is the exclusive member of the a4 integrin subfamily expressed by CLL cells and has recently been identified as a negative prognostic marker in this disease. VLA-4 plays a key role in the retention of hematopoietic progenitors in BM stroma, which is required for normal early B cell development. The second prognostic indicator, CD38, is a promiscuous AbMole D-Pantothenic acid sodium glycoprotein that functions as an ectoenzyme, a surface receptor, and an adhesion molecule. In CLL, CD38 ligation in the presence of IL-2 induces the survival and proliferation of the tumor cells. Furthermore, CLL cells expressing CD38 are thought to have enhanced migratory capacity.