The Shadaloo Intimidation Network is the antagonistic organization in Street Fighter IV. S.I.N is the weapons division of Shadaloo, although many believed it was the remains of the fallen Shadaloo. In fact, it became an independent organization after M. Bison's death in the second World Warrior Tournament, having all operatives of Shadaloo transferred to SIN after this. Their goal is to complete the BLECE project, a bio-weapon that would allow them to conquer the world. It was devastated by the resurrected M. Bison and his enemies, the Street Fighters (Guile, Chun-Li, Abel), though the two parties were unaware of each other's presence.
- Seth: One of a series of 26 bio-humanoids created by SIN, he was the only one who acquired free will, working his way to become the leader of S.I.N. Believing he is superior to humans, he wishes to become ruler of the world and destroy Bison, who came back from the dead;
- Juri Han: A victim of an attack led by S.I.N, she eventually became one of their numbers, having her lost eye replaced by a Tandem Engine. She actually wants to kill both Seth and Bison by herself to take revenge.
- Crimson Viper: Another S.I.N operative, she became the head of the Battle Suit project, but she expects to be reassigned to the BLECE project. Crimson Viper is actually a CIA undercover agent, trying to collect data connecting BLECE and S.I.N.