When chromosomes mis-segregate during cell division, cells lose their balance. The resulting cells are aneuploid, they contain fewer or more chromosomes than usual. Aneuploidy is generally harmful for the cell and characteristic for pathological conditions such as Down syndrome or cancer. Scientists are currently investigating why aneuploidy is so harmful. Presumably, an imbalance of proteins present in aneuploid cells plays an important role in the process. Nevertheless, many questions regarding the origin of aneuploidy and its consequences still remain unanswered.