The HSL11.rMAb monoclonal antibody specifically recognizes CD179b, which is also known as Ig Lambda 5, λ5 or Ig omega. CD179b is a ~23 kDa protein within the immunoglobulin gene superfamily and is a component of the pre-B cell receptor complex (pre-BCR). The pre-BCR is a heterodimer comprised of an immunoglobulin heavy chain (IgH) and a surrogate light (SL) chain, which is composed of non-covalently linked CD179b (λ5) and CD179a (VpreB) proteins. The pre-BCR is expressed during the early stages of B lymphocytes development and its expression is restricted to pre-B cell stage of B cell development, whereas the expression of SL chain is restricted to both pro-B and pre-B cell stages of B cell development. Due to restricted lineage expression, CD179b (λ5) along with CD179a (VpreB) may serve as a useful marker of precursor B cells and can be utilized for immunophenotyping precursor B cell lymphoblastic lymphomas. The pre-BCR plays a role in signal transduction for cell proliferation and differentiation from the proB cell to the preB cell stage, allelic exclusion of IgH chain loci, and Ig light chain gene rearrangements. CD179b (λ5) is required for precursor B cell development and its deficiency could prevent the maturation and survival of the B cell precursors beyond the pre-B cell stage and can lead to B cell deficiency and agammaglobulinemia.