. "Any process that stops or reduces the activity of the enzyme GTP cyclohydrolase I." . . . "negative regulation of GTP cyclohydrolase I activity" . "down regulation of GTP cyclohydrolase I activity" . "down-regulation of GTP cyclohydrolase I activity" . "downregulation of GTP cyclohydrolase I activity" . "inhibition of GTP cyclohydrolase I activity" . . .