According to assistant director Juilan Spillane, most of the Silicon Knights staff working on the game were not X-Men fans and lacked knowledge of the franchise. Whenever employees who were more familiar with Marvel and X-Men lore attempted to inform project leads about errors, they were immediately shot down. Spillane specifically recalled being shown an early build of the game with a boss fight against the Juggernaut (a character who is invincible to all forms of physical damage, only being vulnerable to mental attacks) where his health bar depleted to zero. When asked what he thought, Spillane replied: "There's no way Marvel is going to approve this." The complaint was shrugged off and sent to Activision and Marvel for approval; in response, Marvel sent back several copies of "Marvel Encyclopedia Vol 1 X-Men" for the developers to use as a reference. In spite of this, Juggernaut's X-Gene is still available as a selectable suit and power, despite him not being a Mutant in most Marvel media.