Think of your DNA as a pile of Legos. These Legos come in four different colors—and each one has a corresponding letter: A, C, T, and G. When you pair them together in different ways, you can get all kinds of different humans. Sometimes, they pair in very different ways from most other people.
This can result in genetic mutations. While these differences can often be harmless, sometimes they’re not—resulting in things like diseases and disabilities just because a Lego was put in the wrong spot.
That’s why scientists have long sought after human genome sequencing, the process of understanding the full order of the bases of DNA (those A, C, T, and G Legos). While the first human genome sequence was mapped in 2001, it wasn’t complete or accurate. Luckily, a lot has advanced in the more than 20 years since then.