The PAM sequence is crucial for CRISPR-Cas system function as it is found in viral genomes but not bacterial genomes.

“what is significant about that is that it it is a sequence that is only found in the bacterial viruses genome but not in the bacteria's genome so so when the bacteria acquires a piece of the viruses sequence and sticks it into its own genome one question is couldn't the crisper system go and recognize the virus's own genome now the bacteria yeah sorry the bacteria's genome and able to cut couldn't this thing it's sort of like couldn't you couldn't you turn the weapon on yourself and and cut your own DNA and kill yourself exactly yeah yeah so how does it avoid this self-targeting or autoimmunity against itself and and this is where the Pam sequence comes in”