People can also develop CJD because they carry a mutation of the gene that codes for the prion protein ("PRNP"). This occurs in only 5–10% of all CJD cases. In sporadic cases, the misfolding of the prion protein is a process which is hypothesized to occur as a result of the effects of aging on cellular machinery, explaining why the disease often appears later in life. An EU study determined that "87% of cases were sporadic, 8% genetic, 5% iatrogenic and less than 1% variant."