Zac Efron celebrated his 28th birthday yesterday, but it wasn't all smiles on the actor's special day.

October 18 also marks the birthday of Zac's late cousin Emily Hobson, who shared the same age and birth date with him.

Emily passed away from cancer two years ago, with her death made all the more tragic by the fact doctors had told her she was in remission just a year previously.

The actor took to Instagram yesterday to mark the day, and asked fans for a special "birthday wish" for himself and Emily – a donation to US charity Stand Up 2 Cancer.

 

A photo posted by Zac Efron (@zacefron) on

"Happy birthday to my cousin and best friend, Emily, who passed away from cancer 2 years ago – she was 28 like me," he wrote alongside a childhood holiday photo of the pair. 

"We share the same birthday and this day is forever ours.

"My birthday wish is for you to donate anything you can to @su2c. Thank you everyone for all the birthday wishes. Love you."

Emily was diagnosed with Ewing Sarcoma, a rare form of bone cancer, in 2009. She was 21 at the time, and was studying Film at the University of California.

After her diagnosis, she began working actively to help form a cancer support group in her area for young adult patients. One friend described her as "the true definition of a fighter."

In a 2012 tweet, Zac told fans that Emily helped him "get started" as an actor.

Zac's Instagram post already almost half a million likes, so we're hoping Stand Up 2 Cancer saw a big boost in donations yesterday.