The Cytochrome P450 enzyme CYP1B1 [1] only occurs in cancer cells [2]. When certain phytoallexins such as reservetol and salvestrol are ingested these phytoallexins are converted by the P450 enzyme into picetannol [3], which is fatal to cancer cells but not human cells [4].

[1] http://en.wikipedia.org/wiki/CYP1B1
[2] http://cancerres.aacrjournals.org/content/57/14/3026.short
[3] http://en.wikipedia.org/wiki/Piceatannol
[4] http://www.nature.com/bjc/journal/v86/n5/abs/6600197a.html