In the 1930s, the tobacco was replaced by gum and the cards became more of the focus, as companies such as Goudey and Play Ball produced cards. It wasn't until after WW1 that cards began to be produced by companies on a regular basis, first with Bowman in 1948, then with Topps in 1951. Topps was the only card company from 1956 through 1980 after it acquired Bowman. In 1981, Fleer and Donruss entered the market, as did Upper Deck in 1989. Since the late 1980s, there has been an explosion of card sets, with each of the four card companies producing dozens of sets in each sport under a variety of labels and set names.