CHICAGO (CBS) — When Shari Hart was diagnosed with leukemia last week, her 14-year-old son wanted to do something special.

So, this past weekend he wrote her a simple message: “HI MOM,” with a smiley face inside the O.

Two simple words that had a big impact, because of the effort it took. Her son, Will, didn’t simply write those words in a text message or on a greeting card, he wrote it in giant letters in the snow on the roof of the parking garage across the street from his mom’s room at Rush University Medical Center.

Later, with the help of his dad, Will added a few more words: “GOD BLESS U.”

Shari, 48, is at Rush for about a month for chemotherapy for acute myeloid leukemia. Five simple words from her son and husband went a long way to lifting her spirits, given the way the words were delivered.

“Anyone who came in the room, I said, ‘Look what my son did!’ It was such a proud thing. And then to have my husband gone down and say, ‘Let’s put a little more on’ … who thinks this? My family, apparently,” she said. “It just gives me true joy, and lifts me up. So when you’re feeling a little low, just some simple words, ‘God bless you,’ made me cry.”

She said she could see her son and husband on the roof, waving and blowing her kisses as they added the last three words.

“It was just a true blessing, but I realized soon it wasn’t just for me. You know, it was really everyone on this side of the building could see it. So it became a lot bigger, a lot quicker,” she said.

Hart said the message also has proved uplifting for hospital staffers stressed out by their jobs.

“A lot of people are like ‘I was having a long day at work here, but seeing your message really helped lift my spirits,’” she said.

Shari Hart. (Family provided photo)

Shari Hart. (Family provided photo)

Angela Washek, a nurse at the surgical ICU at Rush, noticed Will’s message — visible to dozens of rooms at the hospital — very early Sunday morning.

“Saturday night we saw people on the roof running around. We didn’t know what was going on and about half an hour later I looked out and there was this big message out there,” Washek said.

She estimated each letter is about two car lengths long.

“It was really heartfelt. It was a lot of effort and it was really cold that night,” she said.

The effort clearly was worth it, given Shari’s reaction.

“So when you’re feeling a little low, just some simple words, God bless you, made me cry,” she added.