Re-evaluation of the probabilities for productive rearrangements on the κ andλloci

V-J arrangements at Ig light chain (IgL) genes occur in resting small pre-B cells. In the absence of cell division, the probability of production kappa and lambda rearrangements is proportional to the output of kappa+ B and lambda+ B cells in bone marrow. The kinetics and probability of productive kappa or lambda rearrangements was assessed in three groups of mice carrying two (wild-type), one or no intact Igkappa gene, and the following conclusion are drawn. Kappa and lambda rearrangements occur independently at different kinetics, and rearrangements are initiated at a time when kappa rearrangements are stopping. The probability of productive kappa and lambda rearrangements per chromosome is calculated to be approximately 60 and approximately 20% respectively. Thus, a kappa gene can attempt rearrangements up to three times per chromosome during B cell development. These findings explain that the observed ratio of kappa+ B/lambda+ B cell production in wild-type mice is 95/5.