Biosynthetically, the biochemical transformation from tryptophan to psilocybin involves several enzyme reactions: decarboxylation, methylation at the N9 position, 4-hydroxylation, and "O"-phosphorylation. Isotopic labeling experiments from the 1960s suggested that tryptophan decarboxylation is the initial biosynthetic step and that "O"-phosphorylation is the final step, but recent analyses of isolated enzymes demonstrate that "O"-phosphorylation is the third step in "P. cubensis." The sequence of the intermediate enzymatic steps has been shown to involve 4 different enzymes (PsiD, PsiH, PsiK, and PsiM) in "P. cubensis" and "P. cyanescens", although the biosynthetic pathway may differ between species. These enzymes are encoded in gene clusters in "Psilocybe, Panaeolus," and "Gymnopilus".