In a pilot study on relapse/refractory MCL patients, activity of PD0332991 was monitored via 3-deoxy-3[18F]-fluorothymidine (FLT) and 2-deoxy-2-[18F] fluoro-D-glucose (FDG) positron emission tomography/computed tomography (PET/CT) imaging as well as with Ki67 staining of the biopsy samples. The gene discussed is MKI67; the disease is mantle cell lymphoma.