If you see this sign on your nails, you need to get checked for cancer immediately.

“A few weeks ago I had a client who comes in occasionally for a manicurist,” Jean Skinner, a manicurist, wrote in a Facebook post. “She had a dark vertical stripe running down her nail. She said as soon as she sat down—I needed a color dark enough to cover the stripe.”

Other salons had speculated that the mysterious streak on the woman’s nail might be a calcium deficiency, a blood clot, or some strange genetic trait. Skinner, however, knew better. She immediately told her client that the dark streak could be a little-known symptom of malignant melanoma, a form of skin cancer.

“I didn’t want to scare her, but I told her she needed to see a doctor right away!” Jean wrote. “She called me today to say it was a very aggressive melanoma that had spread to her lymph nodes.” Unfortunately, the client’s prognosis didn’t look good.

Now, Skinner is using the photo of this female client's nails (with permission) to inform and warn others.

“Please pay attention to abnormalities in your nail bed!!” Jean wrote. “Strange nail changes are most likely nothing to worry about, but sometimes they are a sign of a very serious disease… Early diagnosis can make a big difference.”

Facebook user Lauren Ann had a similar experience. Last year, she shared a photo of her best friend’s fingernails, which showed a similar streak on her nail.

This is a picture of malignant melanoma on the thumb nail. ... One of my best friends wears gel nails regularly, like many of us.

Before she was diagnosed I didn't think much about the dangers of UV light being so close to the finger/nail, especially with some fairly new chemicals on it, she no longer has these nails because thank God she was smart enough to go to a dermatologist and he diagnosed her with a rare form of cancer and had surgery.

While she misses her 10th nail, she's lucky it hasn't spread yet. And I'm incredibly grateful that she caught it early, be cautious when getting gel nails, and consider these types of risks.

And now you're probably looking at your fingernails. If you notice dark spots under your nails, see a doctor immediately.