The fragile site FRA11B has been localized to the p(CCG)(n) repeat of
the CBL2 proto-oncogene. A proportion of Jacobsen (11q(-)) syndrome pa
tients inherited a chromosome carrying a CBL2 p(CCG)(n) expansion, whi
ch was truncated close to FRA11B. These results have broad implication
s for the role of p(CCG)(n) repeat expansion in the aetiology of genet
ic disease involving chromosome rearrangements.