One of the special rules in the Forge World Red Scorpions army list called PURITY ABOVE ALL. The fluff is that the chapter closely monitors it's geneseed, screening for mutation and corruption, to the point of fanatiscism! The game rule allows you to upgrade any Sergeants in Tactical Squads to Apothecaries for 25pts which will give the entire squad FEEL NO PAIN (Ignore unsaved Wounds on a 5+).
In light of the post from yesterday, I should also mention the other special rule for the Red Scorpions - HAVE PRIDE IN YOUR COLOURS. The Red Scorpions shun the use of camouflage and it's attendant tactics such as covert operations, stealth, and infiltration. This means no model in the army may use the INFILTRATION or SCOUT special rule so my Scout units will set up in the same manner as the rest of my troops... still worth it for the Sniper Rifles!