CD179b, also known as Ig Lambda 5, lambda5, and Ig omega, is a 22 kD, C-like protein member within the Ig superfamily. lambda5 associates with CD179a (VpreB) to form a surrogate light (SL) chain that in turn associates with the IgM heavy chain and the CD79a/CD79b heterodimer to form the pre-B cell receptor (PreBCR) complex. The function of CD179a/CD179b is still under investigation, though it has been reported that the deficiency of CD179a and/or CD179b impairs the maturation of the B cell precursors beyond the pre-B cell stage.