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What Exactly Is a Mutation? And What Are All Those Numbers About?
Part of A Guide to HIV Drug Resistance

December 2009

To make it easier to work with HIV resistance tests, researchers have a shorthand system for naming HIV mutations.

An HIV "mutation" is actually just a slight change in a specific section of HIV's RNA, the genetic code that provides all the instructions for how HIV works. Mutations occur naturally, not just in HIV, but in other viruses as well -- not to mention within the cells of every other living thing, humans included. Every HIV mutation is given a unique name to help researchers identify it.

Let's look at K103N, the most common mutation found in people who are taking medications such as Atripla, Sustiva and Viramune. The number in the middle is called a "codon" -- it identifies the specific position within HIV's RNA where the mutation is located. The first letter stands for the amino acid that is normally found at that position in wild-type HIV; the last letter stands for the amino acid that's there instead (which is the mutation).

What Exactly Is a Mutation? And What Are All Those Numbers About?

So, in the case of K103N, we have a mutation at codon 103 in HIV's reverse transcriptase gene. In that particular spot, the amino acid K (which stands for lysine) has been replaced by the amino acid N (which stands for asparagine), which makes that particular copy of HIV into a mutation. It just so happens that this particular mutation, K103N, is one of the worst mutations to have, since it makes HIV highly resistant to many of the approved drugs in the NNRTI class. Not all mutations, of course, are so dangerous. Most mutations will have no impact at all on how well your medications work.

Meet the Mutations: A Class-by-Class Breakdown

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