Ring chromosome. Coloured scanning electron micrograph (SEM) of a chromosome after treatment with etoposide. Chromosomes normally consist of two identical chromatids joined at a centromere, resulting in an X shape. This chromosome has been broken, causing the rearrangement and interchanging of fragments of the chromosome. Etoposide is a drug that is used to treat some types of cancer, mainly testicular and small-cell lung cancer. It prevents malignant (cancerous) cells from spreading by interfering with cell division mechanisms, eventually causing cell death. It also physically damages chromosomes by causing breaks. These aberrations can also be lethal. Magnification: x19,300 at 10cm wide.