Abstract: Methods and systems for increasing stability of a target polypeptide in a serum are described. The methods and systems utilize a fusion protein comprising a single-domain antibody against a serum albumin (SASA), the target polypeptide and optionally a linker. The fusion protein has a significantly prolonged serum half life in comparison with the target polypeptide alone. The SASA fusion tag also facilitates the expression and purification of the fusion protein. This allows direct in vivo screening or utilization of the target polypeptide for its biological activity or efficacy regardless of its intrinsic serum half life, which has significantly increased the number of candidates for the development of novel protein based diagnosis or treatment.