Acknowledgments The authors wish to thank Nicole Däschlein, Hyun-Jin Kim and Rebecca Walser, all at XL-protein, for help in gene cloning and western blotting and Corinna Brandt and Stefan Achatz at TUM for performing MS analyses. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Author Contributions Conceptualization, U.B. and A.S.; experimental work, U.B.; manuscript preparation, U.B. and A.S. All authors have read and agreed to the published version of the manuscript. Funding This research received no external funding. Institutional Review Board Statement The study was conducted in compliance with applicable animal welfare regulations (Institutional Animal Care and Use Committee approval No. 2020_ATRC/5999_F). The permission code for PK studies (PK, metabolism, and bioequivalence) was PE/EA/1374-5/2017 and the permitting authority was the Government Office for Pest County, Directorate of Food Chain Safety and Animal Health, Hungary. Informed Consent Statement Not applicable. Data Availability Statement The data presented in this study are available on request from the corresponding authors. Conflicts of Interest U.B. and A.S. are co-founders and shareholders of XL-protein GmbH. Abbreviations AEX Anion exchange AP Alkaline phosphatase aTc Anhydrotetracycline hydrochloride AUC Area und the curve BCIP 5-Bromo-4-chloro-3-indoxyl phosphate BSA Bovine serum albumin CEX Cation exchange CL Clearance Cmax Maximum (or peak) serum concentration CNP C-type natriuretic peptide CORYNEX Corynebacterium glutamicum protein expression system COVID-19 Severe acute respiratory syndrome coronavirus 2 CTLA-4 Cytotoxic T-lymphocyte-associated protein 4 CV Column volume DCs Dendritic cells DPP-IV Dipeptidyl peptidase-4 ELISA Enzyme-linked immunosorbent assay ESETEC E. coli secretion technology ESI-MS Electrospray ionization mass spectrometry FPLC Fast protein liquid chromatography GCC Glucagon GIP Glucose-dependent insulinotropic polypeptide GLP-1 Glucagon-like peptide 1 GM-CSF Granulocyte-macrophage colony-stimulating factor GSH γ-L-Glutamyl-L-cysteinyl-glycine HBV Hepatitis B virus HCV Hepatitis C virus hGH Human growth hormone hIFN-α Human interferon 1 HIV-1 Human immunodeficiency virus I iRGD Internalizing arginylglycylaspartic acid peptide K3-EDTA Tri-potassium ethylenediaminetetraacetic acid MHC Major histocompatibility complex MOPS 3-(N-morpholino)propanesulfonic acid NBT Nitro blue tetrazolium OD Optical density at 550 nm PAS Proline/alanine-rich sequence PAS-Tα1 N-terminally PASylated Tα1 PBS Phosphate-buffered saline PD Pharmacodynamics PEG Polyethylene glycol p.i. Post injection PK Pharmacokinetics ProTα Prothymosin alpha PTH Parathyroid hormone Q-TOF Quadrupole time of flight RBS Ribosome-binding site RGD Arginylglycylaspartic acid RimJ Ribosomal-protein-S5-alanine N-acetyltransferase RPC Reverse phase chromatography RT Room temperature SEC Size exclusion chromatography SDS-PAGE Sodium dodecyl sulfate polyacrylamide gel electrophoresis τ1/2α Half-life of distribution τ1/2β Elimination half-life Tα1 Thymosin alpha 1 Tα1-PAS C-terminally PASylated Thymosin alpha 1 tetp/o Tetracycline promoter/operator Th T helper cell tlpp Lipoprotein terminator TLR-2 Toll-like receptor 1 TLR-9 Toll-like receptor 2 TMAE Trimethylaminoethyl tmax Time to reach Cmax