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Furin is a human enzyme, a protease. (It chops proteins).

The FCS on the virus is the Furin Cleavage Site.

What?

It’s the place on the virus, that a furin enzyme, can cut the virus.

The specific place on the virus is between the S1 and S2 sub/components of the famous “spike” protein on the virus.

Once the virus is cut there, the virus can join its membrane to the human cell membrane, and empty its viral insides into the human cell.

The insides in this case are the RNA containing the instructions for making *more* viruses.

And that’s how viruses multiply inside the body: they tell a cell to make more and more and more and MORE virus,
till it gives up from exhaustion, or just pops like an overfilled water balloon, or whatever.

So back to the big picture.

Like it was described above, the furin cleavage site is part of a protein in the virus.

As a protein, it is made up of different amino acids (like a sentence, or a paragraph, is made up of letters in a specific order).

So it turns out, the exact order of the amino acids, in the FCS which is part of the *virus*, is the same as a sequence of amino acids, in a completely different protein found in human beings.

(”Hey, how’d that get there?”)

But there’s one more thing.

Remember when I said the interior of the virus contains RNA, which are the instructions for building new virus?

How does this happen?

Well, the instructions work like this: Think of DNA. It’s a double helix (think of a ladder with a spiral twist in it).
You can untwist the ladder, pull each half apart, and start reading the rungs, one at a time. There are four different possible rungs — call them A, T, G, and C. An A on one half of the ladder matches with a T on the other half, and G matches with C.

So if you have half the ladder, you know how to make the other half.

So with DNA, the protein factory inside the cell reads the separated ladder 3 rungs at a time. “A, C, A”...”G, C, T” ...”T, A, G”... etc.
Each set of 3 rungs, tells the cell the next amino acid to tack onto the end of the new protein it is building.

That’s for DNA; RNA is a little different, but the idea of groups of 3 rungs in a row, telling the body what the next amino acid to get, is the same.

Now if you’re bored, you can work out the number of combinations of 3-letters-in-a-row, when you only get to use 1 of 4 letters; and you can (and scientists have) figure out which triplet, tells you to get which amino acid.

All clear so far?

Well, here’s the other part. (And high time, too.)

Even though there is more than one triplet which can code for the same amino acid, it turns out, certain triplets are used most of the time by people; certain other triplets are used (for that same amino acid) by viruses.

So it turns out, not only does the viurus protein itself, right at the FCS, look exactly like the protein in humans from some other part of the human body...but the genetic code for it, FROM THE VIRUS RNA, uses the 3-letter combinations for instructions, that humans use almost all the time, and viruses don’t.

Gee, how’d THAT get there too?

And right at the most important part of the virus for infecting people?

How conVEEEEEENient.