Perhaps you've seen the hashtag alongside mentions of Zayn Malik or Justin Bieber.
Perhaps you've been confused by it: What the heck is #WearYellowforSeth and why is it trending, you may have wondered?
Grab your tissues and scroll down to find out why...
Seth Lane is a five-year old who was diagnosed with severe combined immunodeficiency disorder at five months old. This basically means he was born without an immune system.
He had a bone marrow transplant at seven months, but it did not take and the Northamptonshire, England must now spend most of his time inside a sterile room to be protected from germs and bacteria.
On March 11, Seth's mom, Leanne, posted a video of her son asking the world to wear his favorite color March 27 to raise awareness of this illness.
"Yellow is Seth's absolute favorite color," Lane says. "He loves anything yellow… It perks him up when he's feeling rubbish, really.”
Leanne hopes to post photos of strangers wearing yellow in support of her child around her room, and she also hopes the movement simply spreads awareness about severe combined immunodeficiency disorder.
"If more children can get more matches because of this, even if one child can get a match, then that’s fantastic," Lane tells ABC.
Lane family members are not seeking any donations. They just want people to share pictures of themselves wearing yellow for Seth.
"He is the most loving, happy child," Leanne says of the toddler, who must have his gallbladder removed next week and who is then due for another bone marrow transplant.
"When we went to the hospital, he said to me, 'I'm not going home for a long time Mummy, am I?' I said, 'No, you’ve got a lot to do here' and he comes to the conclusion that he's just accepted it."
