Joe Biden frequently mentions his son, Beau, a former Delaware attorney general who died of brain cancer in 2015 at the age of 46.

Joe and eldest son Beau have always been very close.

After the tragedy, Biden was sworn in as senator at Beau's hospital bed and he nicknamed "Joe the Train" for commuting by train daily between Washington and Delaware (takes around 90 minutes per trip.

"I heal the pain by focusing on the children," Biden said in 2015. "The strong bond between me and my children is a gift I would not have had had I had not experienced that tragedy."

"One of my deepest memories is that when the brothers were hospitalized, Dad was always with us. We were all he cared about, not the Senate," Beau said at the Congress.

Beau Biden attended the University of Pennsylvania and attended law school at Syracuse University, where his father was also an alumnus.

In 2006, Beau ran for Delaware attorney general and won 13,000 votes.

Beau suffered a minor stroke in 2010 and three years later, he was diagnosed with brain cancer.

Biden was at the end of his second term as vice president.

When Biden announced his election for president, he mentioned the tragedies his family went through to emphasize his empathy for those who have suffered hardships and losses and to prove he is someone who can steer the country.

Joe also mentioned Beau when explaining the reasons for choosing California senator Kamala Harris, who has a close relationship with his son, as the vice presidential candidate.

A longtime Biden supporter commented that Beau was Biden's "perfect son", as opposed to brother Hunter, who was caught up in scandals.

"Beau is very easy to work with, very determined but also easy to sympathize with others," said former Massachusetts attorney general Martha Coakley.