Examples of residual HCL in bone marrow specimens. (A) Bone marrow biopsy from patient 10 with MRD showing normocellular hematopoiesis with no morphologic evidence of an abnormal lymphoid infiltrate (periodic acid–Schiff, original magnification ×400). (B) CD20 immunostain of the bone marrow biopsy from patient 10 with MRD showing scattered abnormal B cells. The illustrated cell has a bilobed nucleus, abundant cytoplasm, and bright coarse CD20 positivity, all characteristic features of HCL (CD20 immunostain/hematoxylin, original magnification ×1000). (C) Bone marrow biopsy from patient 19 with gross morphologic disease showed multiple areas of lymphoid infiltrate. The infiltrate was loosely cellular with monotonous small lymphocytes with nuclear features and abundant cytoplasmic domains imparting a “fried egg” appearance characteristic of HCL (periodic acid–Schiff, original magnification ×400). (D) CD20 immunostain of the bone marrow biopsy from patient 19 with gross morphologic disease shows extensive involvement by a B-cell infiltrate with features of HCL. Both patients had their diagnosis confirmed by flow cytometry that showed a CD11c, CD25, and CD103⁺ population of monoclonal B cells.