We are pleased to announce that we have selected the highly cited review papers published in 2021, which you are welcome to read.

1. “Collider Searches for Dark Matter through the Higgs Lens”
by Spyros Argyropoulos, Oleg Brandt and Ulrich Haisch
Symmetry 2021, 13(12), 2406; https://doi.org/10.3390/sym13122406
Available online: https://www.mdpi.com/2073-8994/13/12/2406

2. “Review of Shape Phase Transition Studies for Bose-Fermi Systems: The Effect of the Odd-Particle on the Bosonic Core”
by M. Böyükata, C. E. Alonsom, J. M. Arias, L. Fortunato and A. Vitturi
Symmetry 2021, 13(2), 215; https://doi.org/10.3390/sym13020215
Available online: https://www.mdpi.com/2073-8994/13/2/215

3. “Strong-Interaction Matter under Extreme Conditions from Chiral Quark Models with Nonlocal Separable Interactions”
by Daniel Gómez Dumm, Juan Pablo Carlomagno and Norberto N. Scoccola
Symmetry 2021, 13(1), 121; https://doi.org/10.3390/sym13010121
Available online: https://www.mdpi.com/2073-8994/13/1/121

4. “Digital Twins-Based Smart Design and Control of Ultra-Precision Machining: A Review”
by Lei Wu, Jiewu Leng and Bingfeng Ju
Symmetry 2021, 13(9), 1717; https://doi.org/10.3390/sym13091717
Available online: https://www.mdpi.com/2073-8994/13/9/1717

5. “Interplay between Spacetime Curvature, Speed of Light and Quantum Deformations of Relativistic Symmetries”
by Angel Ballesteros, Giulia Gubitosi and Flavio Mercati
Symmetry 2021, 13(11), 2099; https://doi.org/10.3390/sym13112099
Available online: https://www.mdpi.com/2073-8994/13/11/2099

6. “Atomic Structure Calculations of Helium with Correlated Exponential Functions”
by Vladimir A. Yerokhin, Vojtěch Patkóš and Krzysztof Pachucki
Symmetry 2021, 13(7), 1246; https://doi.org/10.3390/sym13071246
Available online: https://www.mdpi.com/2073-8994/13/7/1246

7. “Evolution of Heavy Ion Beam Probing from the Origins to Study of Symmetric Structures in Fusion Plasmas”
by Alexander Melnikov
Symmetry 2021, 13(8), 1367; https://doi.org/10.3390/sym13081367
Available online: https://www.mdpi.com/2073-8994/13/8/1367

8. “Entropy-Like Properties and Lq-Norms of Hypergeometric Orthogonal Polynomials: Degree Asymptotics”
by Jesús S. Dehesa
Symmetry 2021, 13(8), 1416; https://doi.org/10.3390/sym13081416
Available online: https://www.mdpi.com/2073-8994/13/8/1416

9. “Acyl(imidoyl)ketenes: Reactive Bidentate Oxa/Aza-Dienes for Organic Synthesis”
by Ekaterina A. Lystsova, Ekaterina E. Khramtsova and Andrey N. Maslivets
Symmetry 2021, 13(8), 1509; https://doi.org/10.3390/sym13081509
Available online: https://www.mdpi.com/2073-8994/13/8/1509

10. “A Unified Mathematical Formalism for First to Third Order Dielectric Response of Matter: Application to Surface-Specific Two-Colour Vibrational Optical Spectroscopy”
by Christophe Humbert and Thomas Noblet
Symmetry 2021, 13(1), 153; https://doi.org/10.3390/sym13010153
Available online: https://www.mdpi.com/2073-8994/13/1/153

11. “Atypical Brain Asymmetry in Human Situs Inversus: Gut Feeling or Real Evidence?”
by Guy Vingerhoets, Robin Gerrits and Helena Verhelst
Symmetry 2021, 13(4), 695; https://doi.org/10.3390/sym13040695
Available online: https://www.mdpi.com/2073-8994/13/4/695

12. “Actual Symmetry of Symmetric Molecular Adducts in the Gas Phase, Solution and in the Solid State?”
by Ilya G. Shenderovich
Symmetry 2021, 13(5), 756; https://doi.org/10.3390/sym13050756
Available online: https://www.mdpi.com/2073-8994/13/5/756

13. “Model Membrane Systems Used to Study Plasma Membrane Lipid Asymmetry?”
by Haden L. Scott, Kristen B. Kennison, Thais A. Enoki, Milka Doktorova, Jacob J. Kinnun, Frederick A. Heberle and John Katsaras
Symmetry 2021, 13(8), 1356; https://doi.org/10.3390/sym13081356
Available online: https://www.mdpi.com/2073-8994/13/8/1356

14. “Jacobi Multipliers in Integrability and the Inverse Problem of Mechanics?”
by José F. Cariñena and José Fernández-Núñez
Symmetry 2021, 13(8), 1413; https://doi.org/10.3390/sym13081413
Available online: https://www.mdpi.com/2073-8994/13/8/1413

15. “Progress on the Stereoselective Synthesis of Chiral Molecules Based on Metal-Catalyzed Dynamic Kinetic Resolution of Alcohols with Lipases?”
by Raffaella Ferraccioli
Symmetry 2021, 13(9), 1744; https://doi.org/10.3390/sym13091744
Available online: https://www.mdpi.com/2073-8994/13/9/1744

16. “Lattice Constraints on the QCD Chiral Phase Transition at Finite Temperature and Baryon Density”
by Owe Philipsen
Symmetry 2021, 13(11), 2079; https://doi.org/10.3390/sym13112079
Available online: https://www.mdpi.com/2073-8994/13/11/2079

17. “Symmetry Transformations in Metazoan Evolution and Development”
by Valeria V. Isaeva and Nickolay V. Kasyanov
Symmetry 2021, 13(2), 2915; https://doi.org/10.3390/sym13020160
Available online: https://www.mdpi.com/2073-8994/13/2/160

18. “Recent Developments of the Lauricella String Scattering Amplitudes and Their Exact SL(K + 3,C) Symmetry”
by Sheng-Hong Lai, Jen-Chi Lee and Yi Yang
Symmetry 2021, 13(3), 454; https://doi.org/10.3390/sym13030454
Available online: https://www.mdpi.com/2073-8994/13/3/454