Actor Zac Efron had a bittersweet birthday on Sunday (18Oct15) as he paid tribute to his late cousin who passed away from cancer.

The Neighbors star turned 28 and took to Instagram.com to honour his cousin Emily, who lost her battle with the disease two years ago (13).

Posting a picture of the pair on a beach as kids, Zac wrote, "Happy birthday to my cousin and best friend, Emily, who passed away from cancer 2 years ago - she was 28 like me. We share the same birthday and this day is forever ours."

Zac then encouraged fans wanting to give him a birthday gift to make a donation to Stand Up To Cancer, an organisation which raises funds for cancer research.

"My birthday wish is for you to donate anything you can to @su2c (Stand Up To Cancer)," he added. "Thank you everyone for all the birthday wishes. Love you."