Marvel earned a great deal of money and recognition during the comic book boom of the early <answer>1990</answer>s, launching the successful 2099 line of comics set in the future (Spider-Man 2099, etc.) and the creatively daring though commercially unsuccessful Razorline imprint of superhero comics created by novelist and filmmaker Clive Barker. In <answer>1990</answer>, Marvel began selling Marvel Universe Cards with trading card maker SkyBox International. These were collectible trading cards that featured the characters and events of the Marvel Universe. The <answer>1990</answer>s saw the rise of variant covers, cover enhancements, swimsuit issues, and company-wide crossovers that affected the overall continuity of the fictional Marvel Universe
In what year did Marvel introduce trading cards based on their characters?
1990