In pre-B cells, the mu chain is expressed at the cell surface in association with a "light chain surrogate" encoded by the V pre-B and the lambda-like genes. This mu-psi-L complex presumably triggers early steps of the B cell differentiation, possibly by controlling the Ig gene rearrangements. In the humans, the lambda-like complex contains 3 genes, located in the 22q11.2-q12.3 band, telomeric to the IGCL locus, with which they share a similar organization, pointing to a common genetic origin. Only one lambda-like gene, 14.1, is functional and specifically expressed with V pre-B in pre-B cells. This expression starts in cells which still have the IGH locus in germline configuration (pro-B stage) and ceases as soon as the IGL loci rearrange. These pre-B specific transcripts provide useful markers of cells of the B lineage in both physiological and pathological situations.