Pediatricians are accustomed to caring for patients with rare diseases. But as all physicians know, common diseases can also behave in rare ways, either presenting in unusual forms or responding only to particular treatments. Recent advances in molecular medicine have confirmed this intuition, particularly in cancer, whose varieties can be virtually as unique as we are ourselves.

Oncologists have formed cooperative groups to meet this challenge long ago. But they remain outmatched by cancer itself: its biology is complex, with different tumors growing as a result of hundreds to thousands of aberrations in cancer genomes and cells. And, until recently, they’ve been limited by the process of traditional clinical trials. …