San Francisco 49ers quarterback Colin Kaepernick’s refusal to stand for the national anthem before games as a protest against recent high-profile incidents of police brutality and racial injustice have been met with criticism and protests, but is an important step for a league where professional athletes rarely speak out on such issues.

Kaepernick was noticed sitting down during the playing of “The Star Spangled Banner” in a preseason game Friday. When asked by a reporter about his actions, he said:

I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color. To me, this is bigger than football and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder.

The NFL does not require its athletes stand for the national anthem, and, indeed, the 49ers said it was within Kaepernick’s right to not participate. But some coaches say they expect members of their team to stand, regardless of their personal feelings. It’s a delicate line that Rex Ryan, the Buffalo Bills head coach, addressed in a Sunday news conference:

Anytime I talk to my team about that, if there’s personal beliefs or whatever that keep you from doing it, I understand. But at the same time, you know, you’ve got to look at the gifts that we have, the opportunity that we have to play a great game is through the men and women that serve our country. I think that’s an opportunity right there just to show respect, and I think that’s why when you see our team, every one of us are on that line and that’s kind of our way of giving thanks.

Reaction to Kaepernick’s actions were largely negative and included people burning his jerseys. Pro athletes in other leagues have expressed solidarity with the Black Lives Matters movement. NBA stars famously protested the killings of unarmed black men by white officers, as did their counterparts in the WNBA; even Michael Jordan, who was notoriously taciturn about politics during his playing days, has weighed in. But this is an important moment for professional football: one of the league’s stars is speaking out on social-justice issues, something that is rare in the NFL.

Earlier this month, Aaron Rodgers, the Green Bay Packers quarterback, said more athletes should speak out on critical issues. But stopping them is the NFL’s culture, he said. He told ESPN:

I think some guys in the NFL are probably worried about repercussions on speaking their mind from the league … I think if more guys maybe did in our league, it would create a domino effect possibly.

But could more athletes join Kaepernick’s protest?