Most human tumours have one thing in common: They harbour drastically increased amounts of the so-called Myc proteins. Animal experiments show that such high Myc concentrations contribute to causing cancer. But Myc proteins not only have harmful properties, they are also crucial in healthy cells: Acting as “transcription factors”, they control the activity of a limited number of genes. A lot of these genes activated by Myc are essential for the growth and proliferation of normal cells so that Myc is indispensable for normal human development.