This little-known symptom takes just a second to look for.

High angle shot of happy mother taking picture her baby boy in bed with smart phone at home.Mut Hardman/Shutterstock

Parents love snapping pictures of their kids in both big and small moments. And it’s the worst when your kids won’t cooperate and avoid the camera, or when they have red eyes in a photo. But if you notice that your child’s eye has a white glow, it could be a sign of a rare eye cancer.

Although there are several other potential causes for this “white eye,” a severe one is retinoblastoma—a rare form of eye cancer that affects babies and children under the age of six. The first sign of this cancer is white eye or leukocoria: a white pupil or a reflex from one eye in a flash photograph. The other eye usually has a normal “red reflex,” or red eye, according to Howard R. Krauss, MD, surgical neuro-ophthalmologist and clinical professor of ophthalmology and neurosurgery at John Wayne Cancer Institute at Providence Saint John’s Health Center in Santa Monica, California. White eye might also be visible when your child is in a dark or artificially lit room, according to the Childhood Eye Cancer Trust. These are 50 other health symptoms you shouldn’t ignore.

close up of the leukicoria eyeARZTSAMUI/Shutterstock

Retinoblastoma accounts for about 2 percent of all childhood cancers, and only 200 to 300 are diagnosed each year in the United States, according to the American Cancer Society (ACS). “In most cases, this asymmetric reflex is a sign of some other type of problem warranting attention, rather than cancer,” Dr. Krauss says. Other serious but non-cancerous causes of white eye include Coats’ disease, cataracts, persistent hyperplastic primary vitreous (PHPV), and toxocariasis, according to Dr. Krauss.

On the other hand, white eye isn’t the only symptom or sign of retinoblastoma either. Kids might also have vision loss in one eye, experience redness, irritation or pain in one eye, misalignment of one eye, or a delay in eye growth or development, Dr. Krauss says. Here are 15 things cancer doctors do to avoid cancer.

If you think your child has white eye or is experiencing any painful eye symptoms, it’s a good idea to take them to their pediatrician who should already be routinely checking them for eye problems. Neglected childhood eye issues may become uncorrectable if they go undetected and untreated, Dr. Kraus warns. In fact, early detection is key for kids with retinoblastoma. Nine out of ten children in the United States with the cancer are cured, but the odds aren’t as good if the cancer lingers and spreads outside of the eye, per the ACS, so it’s worth keeping an extra eye on those of your children. Don’t forget to schedule regular doctor visits so a medical professional can check your kids’ eyes too. In the meantime, you’ll want to watch out for these 42 strange symptoms that can signal a serious disease.