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Justin Timberlake's back up singer Nicole Hurst, 39, has died. 

The 40-year-old musician shared a tribute to Hurst, who he referred to as his "sister, Friday on Instagram. The post included videos and photos of the two.

"My heart is so heavy," Timberlake captioned the tribute. "We lost a beautiful soul this week. Nicole lit up every room she walked into. On and off the stage she was a constant source of joy and positivity. Some things feel so unfair and we will never understand why they happen. What I do know is that we were blessed to laugh with her, to travel with her, and to experience her infectious smile and love for a life filled with music. 

"Nicole, it’s not enough to say that I am going to miss you dearly," he added. "Thank you for your light. I will do my best to carry that with me. I love you, my sister. Forever family and forever a TN Kid."

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Justin Timberlake attends the "Runner Runner" premiere in 2013.

Besides singing back up for Justin Timberlake, Nicole Hurst also sang alongside Kelly Clarkson. (Photo by David Becker/Getty Images)

Hurst's cause of death is unknown. 

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The musician had been diagnosed with breast cancer in 2013 before entering remission in 2015. In August of 2019, Hurst revealed the cancer had returned and made it to her brain.

"These pictures aren't throwbacks from 2014, but from this year, literally from the 2nd day in January when my world was once again completely flipped upside down," she wrote on Instagram at the time. "What was discovered was not a cancer recurrence, but breast cancer that somehow found it's way to my brain, forcing me to pull the emergency brake on my life and sent me immediately into whole brain radiation treatment to destroy everything that was trying to destroy me."

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"We all know everybody shares their life's highlight reel, but rarely do we share the 'REAL' reel," she continued. "The reality is, it doesn't matter how amazing your life looks, your background, net worth, or even who you sing with, life does not discriminate and can turn on you at any time!"