ATRA-induced G1 arrest and upregulation of p21^WAFI and dephosphorylation of pRB is not reversed by exogenous IL-6. Untreated C5 cells (1) and C5 cells treated with 10⁻⁶ mol/L ATRA (2), 100 ng/mL IL-6 (3), or both (4) for 48 hours were analyzed by DNA flow cytometry for cell-cycle distribution. Their cellular extracts and immunoprecipitated pRB were analyzed by Western blot (row A, anti-p21; row B, anti-CDK2; row C, anti-RB).