Cyclin D1, a potential proto-oncogene that is overexpressed in many different human neoplasms, is one of the primary regulators of the cell cycle. By regulating the activity of cyclin-dependent kinases, cyclins are proteins that govern transitions through distinct phases of the cell cycle.1 Cyclin D1 exhibits maximum expression in the middle to late G1 phase of the cell cycle following growth factor stimulation. Anti-cyclin D1 has been successfully used and is a promising tool for future research into both cell cycle biology and cancer-related abnormalities. This antibody can be used to differentiate follicular lymphomas (cyclin D1 negative) and chronic lymphocytic leukemia/small lymphocytic lymphoma (cyclin D1 positive) from mantle cell lymphomas. Plasma cell myeloma and hairy cell leukemia can express Cyclin D1 weakly.