It is easy enough to make software do what you want it to. You could tell your email client to recognise and immediately delete any unwanted messages – say, any from your mother-in-law that contain the word “visit”, but not the word “cake”. Now, Zhen Xie from Harvard University and MIT has found a way of filtering undesirable human cells – in this case, a specific type of cancer cell – with similar ease.

Xie has developed a genetic “logic circuit” that prompts cells to kill themselves if the levels of five molecules match those of a cancer cell. Yaakov Benenson, who led the study, says, “In the long term, the circuits’ role is to act like miniature surgeons that can identify and destroy cancer cells.” That is a very long way off, but the study is a promising step in the right direction.

Xie worked with HeLa cells, a common line of cervical cancer cells taken from a tobacco farmer called Henrietta Lacks in 1951. Since then, they have become one of the most important tools in modern medicine. Xie identified five small molecules called microRNAs that act ...