"Yes, I know how the Kaminoans did it. They used our genes against us, the ones that make us bond with our brothers, make us loyal, make us respect and obey our fathers - that's what they manipulated to make us more likely to obey orders. They had to remove what made Jango a selfish loner, because that makes a bad infantry soldier, and you can tell from the Alpha ARCs that the Kaminoans weren't wrong. But there's one thing I don't know yet - and that's how they controlled the aging process. That's the key. They robbed us of a full life span. But we will not be defeated by time, ner vod." ―ARC Trooper Lieutenant N-7, Mereel - in an encrypted transmission to N-11, Ordo
Numbering only twelve, the Null-class Advanced Recon Commandos were the first clones of Jango Fett created by the Kaminoan cloners, and the prototypes for the Grand Army of the Republic. They were deemed a failure by the Kaminoans and were not put into mass-production. However, Mandalorian mercenary Kal Skirata saved them from termination by the Kaminoans and later adopted all of them. Most Nulls were named after Mandalorian warriors.
"Highly intelligent, deviant, disturbed—and uncommandable." ―Orun Wa[src] The Kaminoans attempted to enhance the Fett genes, but only six of the twelve prototype troopers survived the gestation period. They refused to follow orders, and were thus deemed unacceptable. The Kaminoans originally intended to "recondition" or terminate the "defective units", but they were saved from destruction by Kal Skirata, a Mandalorian training sergeant hired by Jango Fett to train clone commandos. Kal volunteered to train them as "black ops" commandos, named them, and taught them their Mandalorian heritage. During training, the Nulls were taught to resist interrogation. Though peerlessly efficient, the Null ARCs were quite unpredictable by clone standards, and loyal only to Skirata. Due to their genetic enhancement, they had an eidetic memory, and slightly more muscular physiques. The Null ARC's were also known as Skirata's private army. Despite the fact that their loyalty and their reliability to the Jedi was constantly in question, the Nulls' unique skills compared to the rest of their brethren was undeniable, and thus they and Kal Skirata were reluctantly tolerated in the Grand Army, if only to ensure their cooperation during the war. The Null ARC Troopers worked in the Special Operations Brigade of the Grand Army. It should also be noted that all Null ARC's were officers, except for Null-12. When asked why he was only a Sergeant by one of the Omega Squad commandos, A'den replied, "I preferred to be an NCO. If it's good enough for Kal'buir, it's good enough for me."
Two Clone Intelligence units, Null-6 and Null-10, were the troopers who located General Grievous's hiding place on Utapau near the end of the Clone Wars.
Though they worked for the Grand Army of the Republic, Kal Skirata had them keep alert for evidence leading to the location of Kaminoan Chief Scientist Ko Sai, so Skirata could find out how to decelerate the aging process so as to give any clone a full life span.
The experience of developing Null ARC troopers contributed much in the production of the Alpha-class Advanced Recon Commandos. During the Clone Wars, the Null-class and Alpha-class ARCs harbored a grudge against each other. Whereas the Alpha-class ARCs saw the Nulls as an embarrassment and shame to Jango Fett's legacy of a fully Republic-loyal clone army, the Nulls viewed the Alpha-class ARCs as even more stubborn than Jango and often referred to Alpha-class ARCs as "ordinary" ARC troopers. However, the highest ranking Null, Captain Ordo, managed to retain a respectable relationship with Alpha Captain Maze, although the two were known to have at least once injured the other.