The antibody labels the MUM1 protein, which is expressed in a subset of B cell in the light zone of the germinal center (representing late stages of B-cell differentiation), plasma cells, activated T cells, and a wide spectrum of related haematolymphoid neoplasma. Of non-haematolymphoid neoplasma only a proportion of melanomas are labeled. The antibody is of value, for example, for the subclassification of lymphoid malignancies. Differential identification is aided by the results from a panel of antibodies. Interpretation must be made within the context of the patient’s clinical history and other diagnostic tests by a qualified patholigist.