In brief: To say battle royale games are popular is an understatement, and that’s especially true on the mobile platform. According to analyst firm Sensor Tower, the genre has grossed over $2 billion on iOS and Android in under two years.

While one might expect Fortnite or PUBG to be sitting at the top of the highest-grossing list, that distinction belongs to Knives Out. The battle royale title from Chinese company NetEase has generated more than $643 million across Android and iOS since its launch in November 2017, only a tiny percentage of which came from the US.

Sitting behind Knives Out is Fortnite, which grossed $630 million. That figure only takes into account the game’s revenue on iOS, though, as Fortnite isn’t on Google Play—the Android version uses a dedicated mobile installer, thereby avoiding Google’s 30 percent revenue cut. It's worth remembering that Epic Games’ title isn’t yet available in China, either.

The rest of the top five is made up of PUBG ($439 million), Garena Free Fire ($253 million), and Rules of Survival ($97 million).

Throughout the first quarter of this year, the five Battle Royale titles generated $476 million, a 63 percent increase compared to the same period a year earlier. PUBG was responsible for most of that money, bringing in $148 million or about 31 percent of the total. Fortnite for iOS was second with $119.4 million, a quarterly decline of 26 percent. Knives Out and Rules of Survival also experienced negative growth compared to the previous quarter.

Sensor Tower blames the three games' QoQ revenue decline on several factors, including the increased competition within the Battle Royale category.

With Apex Legends set to arrive on mobile shortly, expect BR mobile games to start bringing in even more money.