R115777 treatment results in the inhibition of FTase but not GGTase I enzymatic activity in PBMCs from patients with MDS. Blood was collected from patients with MDS before R115777 treatment (baseline) and at various days after the initiation of treatment. PBMCs were then prepared and processed for FTase (A) or GGTase I (B) activity assays, as described in “Patients, materials and methods.” FTase and GGTase I activities are reported as percentages relative to control. Baseline counts per minute were 2100 to 5500 per 20 μg for FTase and 370 to 1100 for GGTase I. NR indicates no response.