Universitat Ramon Llull (URL) 1 article published in JoVE Bioengineering Synthesis and Characterization of mRNA-Loaded Poly(Beta Aminoesters) Nanoparticles for Vaccination Purposes Cristina Fornaguera1, Marta Díaz-Caballero1, Coral García-Fernandez1, Laura Olmo1, María Stampa-López Pinto1, María Navalón-López1, Marta Guerra-Rebollo1, Salvador Borrós1 1Grup d’Enginyeria de Materials (Gemat), Institut Químic de Sarrià (IQS), Universitat Ramon Llull (URL) Here, a simple protocol is presented for producing mRNA nanoparticles based on poly(beta aminoester) polymers, easy to be tailored by changing the encapsulated mRNA. The workflow for synthesizing the polymers, the nanoparticles, and their in vitro essential characterization are also described. A proof-of-concept regarding immunization is also added.