The invention concerns a polypeptide which can be isolated from the Brassicaceae family and which has at least the activity of a hydroxynitrile lyase (HNL). The hydroxynitrile lyase of the invention is the first HNL from the Brassicaceae family. The plants (Arabidopsis) from which this enzyme or its gene is isolated is also described as non-cyanogenic. All HNL-containing plants described so far are cyanogenic plants and so it has until now been assumed that only cyanogenic plants contain hydroxynitrile lyases. Surprisingly, it transpires that a polypeptide (AtHNL) of the invention is (R)-selective. The amino acid sequence gives a theoretical molecular weight of 29.2 kDa for the AtHNL subunit. The calculated molecular mass of the protein of approximately 30 kDa can be confirmed by SDS gel electrophoresis.