Oxidosqualene Cyclase Residues that Promote Formation of Cycloartenol, Lanosterol, and Parkeol We are grateful to Bridget M. Joubert for advice regarding mutagenesis. We thank Elizabeth A. Hart for an authentic parkeol standard, and for chromatographic and spectroscopic information. This research was funded by the National Institutes of Health (grant no. AI 41598) and the Robert A. Welch Foundation (grant no. C-1323). M.M.M. was an American Society of Pharmacognosy Undergraduate Fellow. M.J.R.S. was a Robert A. Welch Fellow and was supported by an NIH Biotechnology Training Grant (grant no. T32 GM08362).