The name says it all. Approximately one in ten thousand people are simply incompatible to select kinds of enhancements. Most of them become utter outcasts or lowlives. Others make the most of it. Some people's condition is worse, some can be treated with special medication - however, this medication is extremely expensive. CIS can also affect those with Nano mods. Despite the slight risk of CIS, users often times chose cyber mods over Bio due to the lower cost easier acquisition and ease of repair.
Jackal is known to have CIS.