There’s a question that everyone can (and should) ask about professional success: What is the most important skill I need to develop to excel in my work? The great thing about this question is that applies in a wide variety of situations. When a young person is just entering the work force. When a person in their 30s or 40s has seemingly plateaued and is wondering how to get to the next level. From another perspective, it applies to employers and managers as well. What qualities am I looking for when I interview someone to hire? Or, with my existing employees or team, who has the most of what really matters?

Quick answers spring to mind. The ability to work with team members or bring out the best in others. A solid work ethic. A dedication to excel. But there may be answer that isn’t so obvious but is in fact crucially important.

Jeff Haden, in an article for Inc.com, pondered this very question and decided to turn to advice from someone who has not only enjoyed enormous business success but also managed hundreds of people: Mark Cuban. Haden quotes Cuban, saying, “The people that tend to work for me a long time not only are smart, not only are driven, not only are learners.

“They understand that the greatest value you can offer a boss is to reduce their stress. The people who tend to think that they are invaluable are typically the ones who create the most stress by creating firestorms and creating drama and making things more difficult for me.

“Anybody who reduces my stress becomes invaluable to me. I never want to get rid of them.”

The interesting thing about Cuban’s perspective is what it says about the character of a person who is able to reduce their boss’ stress. It points to a person who is selfless or is at least thinking about the team’s or company’s success and not merely their own. Ego-driven people often create problems, looking to take credit for a project or subtly putting down their colleagues.

This is a valuable lesson for a person at any point in their career. With a little self-reflection, you can probably identify ways to reduce your supervisor or employer’s stress. Or, if you’re torn between deciding which employee to promote, it’s a great question to ask yourself: Which one of my team members is helping me sleep better at night?