To ask whether the expression of both Igκ and Igλ could be the result of a reduced frequency of rearrangement of the KDE in the patients studied, iRSS‐KDE rearrangements were quantified in the genomic DNA isolated from FACS‐sorted populations of CD19+Igλ+ B cells from SLE patients and healthy donors by quantitative real‐time PCR (qRT‐PCR) (Fig. 5A–C). Here, CD19 is linked to systemic lupus erythematosus.