Detection of Cyclin D1 expression by immunohistochemistry (IHC) with Cyclin D1 (SP4) Rabbit Monoclonal Antibody may be used to aid in the diagnosis of mantle cell lymphomas. Cyclin D1 is a nuclear protein that is involved in regulation of the cell cycle.¹ Cyclin D1 is normally expressed in the G1 (resting) phase of the cell cycle. In normal tissues, cyclin D1 expression by immunohistochemistry is restricted to epithelial tissues, endothelium and some fibroblasts and is not detectable in benign lymphocytes.² Mutations in the cyclin D1 gene can lead to overexpression of cyclin D1 in lymphocytes and deregulation of the normal cell cycle, resulting in malignant transformation. Mantle cell lymphomas typically harbor a t(11;14) (q13;q32) translocation that juxtaposes the CCND1 (cyclin D1) gene with an enhancer of the IgH  gene, resulting in cyclin D1 overexpression. Immunohistochemistry for cyclin D1 is positive in mantle cell lymphomas.^1-3 Immunohistochemistry for cyclin D1 shows a nuclear pattern of staining.