Changes in gene expression and aneuploidy are both common features of cancers and under- or over-expression of single genes can cause aneuploidy. Securin, which is involved in chromatid separation, has transforming activity in vitro and is over-expressed in many tumours. We have used a GFP-based assay for chromosome loss in human cells to investigate the effects of over-expressing Securin. We find that over-expression causes a two-fold increase in chromosome loss rate and can thus be a direct cause of aneuploidy.