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.

Question: What notable businesses are using the office space?
Self-contained rewrite: What notable businesses are using the NBC Tower office space?