Medium composition and conditional production of protease-resistant S3-PrP.A, S3, WT, and S1 cells were grown in DMEM (“O%”) or Opti-MEM for a period of 3 weeks. Cell lysates were digested with 50 μg/ml PK. Opti-MEM.B, as per panel A but with S3-PrP cells grown in different ratios of DMEM:Opti-MEM. C, S3 cells previously grown in Opti-MEM for several passages were grown for an additional 3 consecutive weeks in either Opti-MEM or DMEM. Cell extracts were digested with 50 μg/ml PK. DMEM, Dulbecco's modified Eagle's medium; PK, proteinase K; PrP, prion protein.