I have seen multiple 4521-1s, one of those are Dr. Sherman, he claims the tree should scream, we have skinned it, slowly destroyed genetically related trees for sympathy screams, we have carved mouths into the tree, it does not scream.

How it makes 4521-1s? Simple, it really is simple, it essentially makes the person nearby, think it can scream, and it makes the person try to make it scream, instead of the tree screaming, we get a person screaming.

Note: Keep SCP-999 nearby incase of mental breakdowns during testing.