Every organ seals one door until you carry that organ's gene. The gene waits in a Gene Chamber, guarded by a key guardian mini-boss, somewhere you can reach without passing the sealed door.
A door the host has sealed shut: a mucus plug, a tight junction or a fibrin clot.
One per organ, usually on the way to the boss; otherwise (about a third of the time) in front of the Mutation Chamber
Walking into it says which gene you need
It looks like a plug of the barrier (green mucus, a violet tight junction or a dark-red clot) held in a collar of the gene's colour, with a lock badge; with the gene, the lock springs open as you come within 110 points, the collar lets go and the plug melts away
On the minimap the sealed link is drawn in the gene's colour
Carrying the gene dissolves it for good, on both sides
From the Codex
Tissues seal themselves with barriers: thick mucus layers, and tight junctions that zip neighbouring cells together so nothing slips between them.
Fun fact: Some viruses open tight junctions to spread; others are stopped cold by them.
