What happened

The market was unkind to many stocks on Thursday, and that big list of the unfavored included Alphabet (GOOG 0.74%) (GOOGL 0.55%). Yet it was more than just gloomy investor sentiment that sent the company's shares down by 1.4% on the day; a long-serving executive at a key business unit also stepped down.

So what

The person heading for the exit door is Susan Wojcicki, CEO of Alphabet's YouTube. On Thursday, she announced that she was leaving the company.

Wojcicki's departure will surely have quite an impact. She is a personal friend of Larry Page and Sergey Brin, the founders of what is now Alphabet, and has been at the company since 1999 as one of its earliest employees. She became CEO of YouTube in February 2014. Her tenure at Alphabet and her stint as YouTube chief executive are lengthy compared to the frequently brief C-suite stays in top tech businesses.

In a letter to employees that was published on YouTube's official company blog, Wojcicki wrote that she was departing YouTube in order to focus on her family, health, and personal projects. She provided no details, but did add that she will serve as an advisor to Alphabet and Google.

Now what

The transition to new leadership seems to be a smooth one. Wojcicki is being replaced as CEO by YouTube's chief product officer, Neal Mohan, who has served in the latter position since November 2015.

Prior to that, according to his LinkedIn page, he was senior vice president of display and video ads at Google; he started at the company in March 2008.