To the Editor: Rio Frio et al. (Dec. 30 issue)1 report on a germline homozygous mutation in the spindle-assembly checkpoint gene BUB1B in a patient with a nonclassic form of the mosaic variegated aneuploidy syndrome, resulting in reduced expression, increased levels of constitutive aneuploidy, and multiple gastrointestinal neoplasms starting at 34 years of age. The spindle-assembly checkpoint complex, which controls proper chromosome segregation, is sensitive to gene dosage.2 Interestingly, we recently encountered a monoallelic deletion, approximately 1.75 Mb, encompassing another important spindle-assembly checkpoint component gene, BUB1, in a patient with a microsatellite-stable colon carcinoma at 37 years of age. Also, . . .