We sought to develop a more efficient and sustainable route to 1 that could be implemented on scale, where modification of the base to install the N-hydroxy unit is achieved catalytically using an engineered enzyme. Since N-hydroxy-cytidine 4 has previously been shown to undergo lipase catalyzed acylation at C-5-OH , this approach would provide an integrated biocatalytic process to Molnupiravir 1 from readily available starting materials. Our desired route requires a cytidine aminotransferase capable of selectively installing the N-hydroxy unit from NH 2 OH in a bulk water phase (Fig. ). To meet the demands of a robust industrial biocatalyst, we set ourselves the target of developing an engineered enzyme capable of operating at [S] >100 g/L and achieving >90% conversion within 12h using low catalyst loadings to drive down the enzyme cost contribution to the overall process. At the outset of the project there was no known enzyme for this transformation. However, we were intrigued by a report which demonstrated that N-hydroxy-cytidine 4 could be hydrolysed to uridine 3 by wild-type cytidine deaminase, albeit at a very low rate (ca. 2% of activity compared to cytidine 2) . Cytidine deaminase (CD; EC 3.5.4.5) is a zinc containing hydrolytic enzyme which catalyzes the conversion of cytidine 2 to uridine 3 . The enzyme is widely distributed amongst organisms and is involved in the salvage of cytidine for uridine mono-phosphate synthesis. Interestingly wild-type cytidine deaminase from E. coli has been used on scale for hydrolysis of the nucleoside analogue 2'-deoxy-3'-thiacytidine and hence it appeared to be an attractive choice as a potential industrial biocatalyst . On the basis of these observations we initiated experiments aimed at converting cytidine 2 to N-hydroxy-cytidine 4 catalyzed by CD. Cytidine 2 is converted to N-hydroxy-cytidine 4 by an engineered cytidine aminotransferase followed by acylation using Novozym®-435 to give Molnupiravir 1 .