Cluster of Differentiation 74 (CD74), also known as the MHC Class II-associated invariant chain (II), is a type II transmembrane protein. In normal tissues, CD74 is expressed in B cells, monocytes, macrophages, Langerhans cells, dendritic cells, subsets of activated T cells, and thymic epithelium. Under inflammatory conditions, CD74 expression may be observed in endothelial and certain epithelial cells. CD74 expression has been observed in ~90% of B-cell cancers evaluated, as well as the majority of cell lines derived from these cancers. CD74 is a marker for distinguishing atypical fibroxanthoma from malignant fibrous histiocytoma, and Small Cell Lung Carcinoma (SCLC) from Non-Small Cell Lung Carcinoma (NSCLC). It is also a useful marker for distinguishing leiomyosarcoma from leiomyoma. CD74 is clinically indicated as a marker for germinal center lymphocytes and B-cell lymphomas.