HCL expresses B-cell markers such as CD19, CD20, CD22, FMC-7, and CD79a. Although immunohistochemistry on bone marrow biopsy sections is not essential for diagnosis in routine cases, it might be helpful in highlighting the tumor cell population when the marrow involvement is subtle. B-cell markers such as CD20 (shown here) are helpful in distinguishing HCL (which will show strong positivity) from conditions that enter into the differential diagnosis such as myelodysplastic syndrome or aplastic anemia. Immunostaining with antibodies to DBA.44 or CD20 is also useful in assessing minimal residual disease following therapy. Image courtesy of: Jared Block, MD, Carolinas Pathology, Charlotte, NC; Sciplicity, LLC.