When a young person develops cancer, doctors most often assume that genetics are the reason, because the patient hasn’t lived long enough to accumulate environmental damage. But it’s been hard to find the faulty DNA behind many tumors. Now, using new genomic technology, scientists have discovered a novel explanation for some testicular cancers, the most common cause of cancer in men under 35. Rather than being triggered by a single gene mutation, the tumors are caused by too many or too few copies of a gene in a person’s cells. These “copy number variations” have been linked to other conditions such as autism, but never before to cancer.