Original
CDKL5 is a genetic mutation the is de novo- (random spontaneous mutation) meaning it is not (typically) passed from a parent to a child- it happens sometime during fetal development/conception, but scientists have been unable to pinpoint the exact time the mutation occurs.
In extremely rare cases it can be caused by Germaine mosacism - meaning it is in a parents egg or sperm but not in the parent him/herself just in the genetic makeup of the sperm or egg