5 Things to Know This Morning

— -- Your look at the five biggest and most buzz-worthy stories of the morning.

1. Hillary Clinton on Private Email: ‘That Was a Mistake. I’m Sorry’

Hillary Clinton on Tuesday told ABC News’ David Muir that using a personal email account while Secretary of State was a “mistake” and that she is “sorry” for it.

“I do think I could have and should have done a better job answering questions earlier. I really didn’t perhaps appreciate the need to do that,” the democratic presidential candidate told Muir in an exclusive interview in New York City. "What I had done was allowed, it was above board. But in retrospect, as I look back at it now, even though it was allowed, I should have used two accounts. One for personal, one for work-related emails. That was a mistake. I’m sorry about that. I take responsibility.”

2. Zach Anderson, 19-Year-Old Registered Sex Offender, Has Sentence Vacated

Zach Anderson, an Indiana teen who was ordered to spend the next 25 years on the state sex offender registry, had his criminal sexual conduct sentence vacated.

Judge Dennis Wiley, the same judge who sentenced Anderson after he pleaded guilty, granted the motion brought by Anderson’s legal team for a re-sentencing, the Berrian County Court told ABC News. In granting the motion, Wiley recused himself from the case. A bond hearing is scheduled for this Friday, his father Lester Anderson told “Nightline” today.

3. Clerk Who Denied Gay-Marriage Licenses Goes Back to Work

After a five-day stint in jail for refusing to issue marriage licenses to same-sex couples, Rowan County Clerk Kim Davis will return to work this week to face another day of reckoning.

The apostolic Christian, now a symbol of strong religious conviction to thousands across the globe, would not say Tuesday whether she will allow licenses to continue to be issued or try to block them once again, defying a federal court order that could send her back to jail.

Davis walked out of the Carter County Detention Center's front door Tuesday, arm-in-arm with her lawyer and a Republican presidential candidate as thousands of supporters cheered and waved white crosses backed by a 150-voice church choir. Some in the crowd sang "Amazing Grace" and "God Bless America."

4. Serena Williams Beats Venus at US Open to Extend Slam Bid

For stretches of the 27th edition of Williams vs. Williams, Venus gave Serena all she could handle.

And when Serena took control down the stretch to emerge with a 6-2, 1-6, 6-3 victory in a well-played U.S. Open quarterfinal, allowing her to continue pursuing the first calendar-year Grand Slam in more than a quarter-century, a smiling Venus wrapped both arms around her little sister for a warm hug at the net and whispered, "So happy for you."

"Obviously we are very, very tough competitors on the court," Serena said later, "but once the match is over, the second it's done, you know, we're sisters, we're roommates, and we're all that."

5. 'Welcome Back, Kotter' Cast: 40 Years Later

"Welcome Back, Kotter" debuted on September 9, 1975, featuring Gabe Kotter (Gabe Kaplan) returning to his Brooklyn high school to teach a new class of delinquents. The show was wildly popular during its years on the air from 1975 to 1979.

The stars of the show, including John Travolta, went on to have interesting and diverse careers.