Hub was Lan's identical twin brother who died shortly after birth, meaning that they originally shared the same DNA. Yuichiro feared that if the DNA were the same in MegaMan, any damage received by MegaMan would affect Lan, and vice-versa. As a result, Hub's DNA was changed by 0.001% to prevent complete Full Synchro with Lan Hikari. (The 0.001% is presumed to be eye color, as Lan has brown eyes, but Mega Man has green eyes.) When the Hub.BAT is added to MegaMan, the 0.001% difference is removed and MegaMan's code becomes 100% in synch with Lan's DNA, enabling them to go Full Synchro.