In 1951, US surgeons obtained a biopsy from a patient called Henrietta Lacks. Eight months later she was dead of cancer. But the cells from her biopsy, known as HeLa cells, are still growing in culture more than 40 years later. The first established human cell line, they have contributed massively to molecular and cell biology. One scientist argues that they are in effect a new form of life. Today, cell lines are a major tool in medical research.