The NCAA Tournament reaches its regional semifinal stage Thursday with all four No. 1 seeds alive and just seven conferences still represented. Half of the 16 schools remaining in the running for the national championship are from the Big East or Big Ten and teams from each of those leagues will be involved in three of Thursday's matchups. The action begins in Boston at 7:15 p.m. EDT Thursday with the first of the East Region semifinals, one that will send No. 1 seed Syracuse against No. 4 Wisconsin. That game will be followed by second-seeded Ohio State taking on No. 6 Cincinnati. In the West Region, being contested Thursday in Phoenix, No. 1 Michigan State will face No. 4 Louisville and No. 3 Marquette will take on Florida. Friday's games will take place in Atlanta and St. Louis. No. 3 Baylor will open the South Region doubleheader in Atlanta with a game against No. 10 Xavier, after which No. 1 Kentucky will meet No. 4 Indiana. Indiana handed Kentucky its only regular-season loss. No. 1 North Carolina takes on No. 13 Ohio in the Midwest Region opener, followed by No. 2 Kansas against No. 11 North Carolina State. Schools from 31 conferences were in the field when the tournament began last week. Now there are four teams left from the Big Ten and Big East, two each from the Southeastern, Big 12 and Atlantic Coast and one each from the Atlantic 10 and Mid-American. Of the conferences that had more than one entrant in the tournament, the Big Ten has the best record at 9-2. The Big East stands at 11-5. Fourteen of the 31 conferences in the event failed to produce a victory.