 Based on sales data from Amazon.com, Portal 2 was the best-selling game in the United States in the first week of its release, but was overtaken by Mortal Kombat in its second week. According to NPD Group, Portal 2 was the second-best selling game in the U.S. in April 2011, at 637,000 copies, and the fourth-best selling in May. However, NPD does not include sales on Valve's Steam platform. Portal 2 was the best selling game in the U.K. in the first week of its release, the first number-one for a Valve game. It retained the top spot during its second week.
