An abnormal isoform of prion protein (PrP(Sc)), which is composed of the same amino acids as cellular PrP (PrP(C)) and has proteinase K (PK)-resistance, hypothetically converts PrP(C) into PrP(Sc). To investigate the region important for PrP(Sc) production, we examined the levels of PrP(Sc) in PrP gene-deficient cells (HpL3-4) expressing PrP(C) deleted of various regions including the octapeptide repeat region (OR) or hydrophobic region (HR). After Chandler or Obihiro prion infection, PrP(Sc) was produced in HpL3-4 cells expressing wild-type PrP(C) or PrP(C) deleted of HR at an early stage and further reduced to below the detectable level, whereas cells expressing PrP(C) deleted of OR showed no PrP(Sc) production. The results suggest that OR of PrP(C) is required for the early step of efficient PrP(Sc) production.