Fig. 1. Tax maintains proliferation and inhibits differentiation of muscle cells. A, phase-contrast micrographs of C2.7-Neo myoblasts stably expressing the neomycin resistance gene (a and c) and C2.7-Tax myoblasts stably expressing both the neomycin resistance gene and the HTLV-1 Tax protein (b and d). Cells were cultured in proliferation-promoting medium (PM; DMEM supplemented with 10% FCS; a and b). At 80% confluence, they were moved to differentiation medium (DM; DMEM containing 2% horse serum) and cultured for an additional 72 h (c and d). Cells were fixed and stained in a May-Grünwald-Giemsa solution. B, parental C2.7 (), C2.7-Neo (), and C2.7-Tax () cells were seeded at 104 cells in 10-cm diameter dishes and cultured either in proliferation medium (left panel; PM) or in differentiation medium (right panel; DM). At daily intervals, cells detached by trypsinization were counted by a hemocytometer. Data are presented as the cell number (log scale) relative to the time of culture in days.