The B mutation is associated with a tandem duplication of 16A1-16A7. It is unstable, mutating to wild type and to a more extreme form at a frequency of one in 1000 to 3000. The reversion to wild type is associated with the loss of one copy of the duplication, whereas the mutation to extreme B is associated with a triplication of the region. The instability of B has been attributed to unequal crossing-over between the two copies of the duplication. Recent molecular data show that there is a transposable element, B104, between the two copies of the duplication and support the hypothesis that this element generated the duplication via a recombination event. These data suggest that unequal crossing-over within the duplication may not be the cause of the instability of B. Instead, the instability may be caused by a recombination event involving the B104 element. This issue was addressed using probes for the DNA on either side of the B104 element at the B breakpoint. All of the data indicate that the B104 element is not involved in the instability of B and support the original unequal crossing-over model.