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Hank: Henrietta Lacks was born exactly 96 years ago today, on August 1st, 1920 in southern Virginia. She died of cancer at the age of 31. She was a poor tobacco farmer and mother of five, but she was also indirectly responsible for one of the biggest medical breakthroughs of all time. Because a part of her lives on today, in the form of cells that are used for research all around the world. And scientists recently figured out a part of what makes her cells so special.

In 1951, Lacks went to Johns Hopkins hospital, the only hospital in her area that treated African-American patients back then, to be treated for an aggressive form of cervical cancer. While he was examining her, the doctor took a tissue sample from her tumor to use for his research. He didn’t ask her for permission, or tell her how her genetic sample might be used, which isn’t really surprising, since informed consent wasn’t standard practice at the time.

Then he gave her cells to a Johns Hopkins tissue specialist, who looked at her cells under a microscope and found something he’d been searching for for years. See, most cells typically divide 40 or so times before dying. But Lacks’s cells just kept going. In a controlled environment in the lab, some of her cells kept dividing with no signs of slowing down.

It was the first so-called “immortal” cell line, a population of cells with a mutation that allows them to grow and be kept alive for a really long time. 65 years later, the cells are still going. Other immortal cell lines have since been developed. Some of them, like the cells that came from Lacks, have mutations that showed up naturally. Others were engineered on purpose.

But ever since the original HeLa cells, named after the first two letters of Lacks’s first and last names, were put into mass production, they’ve allowed generations of scientists to perform experiments and critical research in cell biology without having to use an actual living person.

And that was, and continues to be, a very big deal. Even though they’re cancerous, and genetically different from typical human cells, HeLa cells still behave like normal cells in a lot of important ways, so they’re incredibly useful for studying things like bacteria, hormones, and especially viruses. Since the early 1950s, researchers have been subjecting HeLa cells to all sort of viruses, both to study how the cells reacted, and to develop life-saving vaccines.

Take polio, for example. Before HeLa, researchers mostly studied it by infecting Rhesus monkey cells with the virus to measure and harvest antibodies. But it was impossible to get enough of those monkey cells to properly test potential vaccines in a reasonable amount of time. So Jonas Salk used HeLa cultures instead, and was eventually able to create, test, and mass-produce his famous polio vaccine.

Since then, billions and billions of HeLa cells have been created, enough cells to wrap around the Earth at least three times. They’ve helped researchers create other vaccines, and been used to develop treatments for diseases like herpes, Parkinson’s, and some kinds of cancer. They’ve also been used to study things like cloning, genetics, and the effects of radiation.

There are tens of thousands of papers that are based on HeLa cells in some way. But for decades, researchers couldn’t figure out exactly what allowed HeLa cells to keep dividing. They knew Lacks’s cancer was in part caused by human papillomavirus, or HPV, and some suspected that her DNA reacted with that virus, or maybe the syphilis she had, making her cells go haywire.

But a lot of people in the 1950’s had some combination of HPV, syphilis, and cancer, and their cells didn’t act like HeLa cells. It wasn’t until 2013, when researchers from the University of Washington sequenced the HeLa genome, that they started to get some answers. The paper’s publication was delayed by a bit while the group got permission to publish from Lacks’s family, a first, since they’d never been involved in HeLa research before.

The researchers found that a scrambled part of the HPV genome did actually combine with Lacks’ genome. Specifically, they discovered that the virus, which contains its own cancer genes, took up shop next to one of Lacks’s oncogenes, genes that can cause cancer when they’re changed. It seems that the HPV essentially turned on that oncogene, which made Lacks’s body produce tons of those cancerous cells. That’s probably at least partly why both Lacks’s cancer and her immortal cell line have been so persistent. And so useful. So happy birthday, Henrietta Lacks, and thank you for everything.

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