The first event of Season Four, Critical Countdown, challenges players to collect DNA samples from fallen enemies in Call of Duty: Modern Warfare Three. This guide will walk you through everything you need to know to collect DNA samples and get the most out of the event.

The DNA samples are similar to dog tags from Kill Confirmed in Call of Duty: MW3, but without a fixed timer, allowing you to collect as many as you want. There are two types of DNA Strands: Ade and Cyto, and you can pick them up from fallen enemies in multiplayer, or loot them from caches and downed enemies in Warzone.

If you’re a zombie enthusiast, head to Urxiklstan and start decimating zombies to collect DNA samples. For the week one challenge, you only need to collect the two types of DNA. However, next week a new type of DNA strand will unlock, rewarding you with sizeable double XP tokens and emblems to add to your collection in Season Four of Modern Warfare Three.

Since there are two types of strands, the most common being blue and the rarest being orange, you’ll need over 40,000 strands to complete the Critical Countdown event. The best way to collect DNA samples is to head over to the Small Map Moshpit playlist and mob players. The Double XP weekend is also live, so you can kill two birds with one stone.