Although originally believed to represent a tumour of haemopoietic progenitor reticuloendothelial cells [2], immunophenotyping identified HCLc as a tumour of mature B cells, typically expressing CD19, CD20, CD200, Tbet, PD1, and four markers of diagnostic value: CD11c and CD103, components of integrin receptors, and CD25 and CD123, components of interleukin receptors, of which at least three are expressed in all cases. The gene discussed is CD19; the disease is neoplasm.