Conversation so far:
Are there any other interesting aspects about this article?
On January 7, 1949, NBC station WNBQ commercially debuted its television broadcast schedule on channel 5.
What did this have to do with Merchandise Mart?
Although WMAQ-TV has since moved to NBC Tower about a mile away, and for the most part the 19th floor of the Mart has been turned into office space.
What notable businesses are using the office space?
one former tenant (Bankers Life and Trust Company) maintained a remnant of the original studios as their video and multimedia department.
Why did WMAQ leave the Mart?
WMAQ-TV has since moved to NBC Tower about a mile away.
Are there any other television companies occupying the mart?
Local regional sports network Comcast SportsNet Chicago has their control room, and broadcasts their live studio programming from the Apparel Center expansion.
Is this the only television company there?
the studios had been home to previous RSNs FSN Chicago and SportsChannel Chicago.

Question: What is the most notable television company to have occupied space in the Mart?
Self-contained rewrite: What is the most notable television company to have occupied space in the Mart?