l-Arg starvation arrests T cells in G₀-G₁ phase of cell cycle. (A) Human T cells (5 × 10⁵) were stimulated with anti-CD3 plus anti-CD28 in the presence or the absence of l-Arg, and cell cycle was assessed at 48 and 72 hours by flow cytometry using propidium iodide. Percentages of T cells at each cell-cycle phase from 5 different experiments were determined. (B) Representative experiments from panel A. Unstimulated control T cells (NS, left row) were in medium containing l-Arg.