Heavy chain diseases (HCDs) are characterized by production and secretion of mutated immunoglobulin heavy chains. Subtypes are alpha, gamma, and mu. Associated with B-cell proliferation. Mu-HCD (the rarest of the 3) classically presents as chronic lymphocytic leukemia, although it has been associated with other clinical presentations such as multiple myeloma. Common findings include splenomegaly, hepatomegaly, and lymphadenopathy. Patients may present with fatigue and pallor from anemia, weight loss, and fever. There may be bone involvement manifesting as pain and pathologic fractures. It typically affects adults over 50 but has been reported in teenagers and elderly patients as well. Prognosis varies.