Major League Baseball Commissioner Rob Manfred said Tuesday the 30-team organization is open to expansion and has compiled a list of candidate cities for potential new clubs.

Speaking ahead of the Major League Baseball All-Star Game at Cincinnati, Manfred did not give a timetable or specify the full list but indicated a willingness to grow the sport by adding teams.

No expansion teams have joined either the American or National leagues since 1998, when the Arizona Diamondbacks and Tampa Bay Rays first took the field.

Expansion candidates could include options in Mexico and Puerto Rico, where the sport is very popular, and Montreal.