Tipifarnib treatment results in inhibition of FTase, but not GGTase I, enzymatic activity in bone marrow cells from patients with advanced multiple myeloma. Bone marrow cells from patients prior to tipifarnib treatment (B) and during the 3rd week of treatment (T) were prepared and processed for FTase (A) or GGTase I (B) enzyme activity assays as described in “Materials and methods.” FTase and GGTase I activities are reported as the percentage relative to control. Control baseline cpm values ranged from 1228 to 3707 for FTase, and from 200 to 2220 for GGTase I. PD and SD designate progressive disease and stable disease, respectively.