DD-426
DD-426 is the program code Tetsuya Ida gives to Ryoko Shinonome, supposedly to greatly increase the Sentinels' capabilities. Shinonome ends up installing this into Ei Sekigahara's Sentinel.
The nanomachine equivalent of a virus that attacks the brain. It severs a portion of the nanomachine that's physically connected to the cerebral cortex, which handles the memory. As the connections get severed, the brain suffers damage, causing severe pain and memory loss. In the worst case, the afflicted are left as mindless husk.
Upon discovering that this program code could be used to sever the Deimos Code, Tsukasa Okino tested the code on himself one year ago. However, he suffered gradual memory loss, so he had no choice but to become an AI with a simulated personality.[1]
Morimura and Renya Gouto try to get Shinonome to remember the password that disables DD-426, but it results in failure and causes her to lose all her memories.
In 1945, DD-426 was successfully disabled thanks to Okino's nanomachine reconstruction and Keitaro Miura's efforts.
Ida uses DD-426 in hopes that the Sentinel Project would result in a failure. He had given up on this world, so he wanted to start a new loop quickly.