Combination of ribociclib and 5-FU is synergistic in inhibiting the proliferation of some RCC cell lines. Combination studies were performed using the Chou-Talalay method. Briefly, the IC50 of the single drug was determined in the single-arm experiments. Combination studies were performed by treating cells with a single drug alone and an equipotent constant-ratio (according to IC50) combination of two drugs. The combination index (CI) was calculated using the CalcuSyn software. CI of less than 1 indicates synergism, CI equal to 1 indicates additivity, and CI of greater than 1 indicates antagonism of the two drugs in combination.