Dear Dan, As mentioned in the grey book, any time one suppresses crossing over on one chromosome it tends to increase crossing over on the other chromosomes. When multiple balancers are used the frequency of exchange on the balancers can increase to the point of seeing survivors come out of such exchanges. FM7 has a relatively large interval, (4E1-11F2), that is unbroken and in correct orientation with respect to the rest of the chromosome. You may see some exchange in this interval. It is usually best to try to use a balancer with breaks closest to the locus that is to be maintained if possible. Good Luck, David O.