Cancer originates with a single affected cell, but it’s been nearly impossible to pinpoint that first malignant mutation. Until now. Scientists have found a way to peek at the very beginning of a melanoma. The key? Tiny, translucent zebrafish.

Here’s what Dr. Leonard Zon of the Harvard Stem Cell Institute said about the study, published in the new Science.

What did you find present at the origin of melanoma?

We found a gene that was present in an embryo that would ultimately make the skin cells, melanocytes, but would also make the tumors, or a melanoma. That gene is on in an embryo and it is shut off and never normally comes back in an adult. But when we see a tumor form in an adult, that gene reemerges.

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What did you do to look at that gene?

We took that gene and fused it to a fluorescent so when the gene is turned on, the cell becomes fluorescent. We then followed fish and we were able to find little tiny patches of a few fluorescent cells. And when we saw these patches come up, 100 percent of the time, that patch went on to form a cancer.

What does that tell you about how the cancer started?

This fluorescent beacon we created was actually reporting this return of those cells to an embryonic or stem cell-like state. What it meant is that the initiation of the cancer was actually turning on this gene, and that’s how the cancer started.

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