IGH split-signal FISH. The percentage of split IGH signals (separate green and red signal together with a colocalized red/green signal) was determined on sorted pre-B-I cells of 3 NBS patients and a healthy donor (A) and on sorted mature B cells of 10 healthy donors and 9 NBS patients (B). The difference in percentage of split signals in mature B cells of NBS patients and healthy donors was statistically significant (P < .001).