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Unspecific peroxygenases (UPOs), the extracellular enzymes capable of oxygenating a potpourri of aliphatic and aromatic substrates with a peroxide as co-substrate, come out with a new reaction: carbon-chain shortening during the conversion of fatty acids with the well-known UPOs from <i>Coprinopsis cinerea</i> (r<i>Cci</i>UPO) and <i>Cyclocybe</i> (<i>Agrocybe</i>) <i>aegerita</i> (<i>Aae</i>UPO). Although a pathway (Cα-oxidation) for shortening the hydrocarbon chain of saturated fatty acids has already been reported for the UPO from <i>Marasmius rotula</i> (<i>Mro</i>UPO), it turned out that r<i>Cci</i>UPO and <i>Aae</i>UPO shorten the chain length of both saturated and unsaturated fatty acids in a different way. Thus, the reaction sequence does not necessarily start at the Cα-carbon (adjacent to the carboxyl group), as in the case of <i>Mro</i>UPO, but proceeds through the subterminal (ω-1 and ω-2) carbons of the chain via several oxygenations. This new type of shortening leads to the formation of a dicarboxylic fatty acid reduced in size by two carbon atoms in the first step, which can subsequently be further shortened, carbon by carbon, by the UPO Cα-oxidation mechanism.