TI - Results PHOSphorylated pRb (Ser780) expression in vivo . AB - Cyclin D1 is characterized as the major D-type cyclin controlling G1-phase progression in kidney cells , both during normal and abnormal proliferation [22 , 23] . To determine whether cyclin D1 may be active in promoting G1 - S progression in kidneys expressing HIV-1 genes , we utilized a well-characterized HIV-1 transgenic mouse model ( Tg26 ) of HIVAN [11-13] . Secondary to renal expression of the HIV-1 NL4-3 Deltagag-pol proviral transgene , adolescent Tg26 transgenics develop a progressive , proliferative renal disease marked by podocyte and tubular epithelial hyperplasia that is indistinguishable both clinically and histopathologically from human HIVAN . In order to determine if cyclin D1 plays a role in this hyperplasia , kidneys from three proteinuric 40-day old transgenic siblings and three non-transgenic littermates were stained by immunohistochemistry with antibody to phosphorylaTED pRb (Ser780) , a site of cyclin D1K-4/6 PHOSphorylation not targeted by cyclin EK-2 complexes that disrupts pRb/E2F binding for G1 - S progression [24] . Figure 1 depicts the intense , heterogeneous nuclear staining of glomerular podocytes and tubular epithelium from hyperplastic nephrons in the diseased transgenics versus the absent staining of nephrons in the non-transgenics consistent with the very low mitotic index in normal kidneys of adult animals [26] . This suggests that cyclin D1 is promoting cell -cycle progression during the proliferation of epithelium in diseased kidneys .