The San Francisco 49ers host the St. Louis Rams on Sunday to close out the 2015 regular season, and the game is going to air on CBS instead of FOX. This slipped under the radar to some extent, but when the league formally flexed Packers-Vikings into Sunday Night Football, they also moved 49ers-Rams to CBS, and Raiders-Chiefs to FOX. Both games kick off at 1:25 p.m. PT.
Normally, FOX hosts NFC games (and inter-conference games hosted by AFC teams), while CBS hosts AFC games (and inter-conference games hosted by NFC teams). However, the league recently expanded to flexing option to allow games to be flexed between networks.