The tumor suppressor protein p53, known as the "guardian of the genome," is regulated by a complex network of post-translational modifications. Phosphorylations at 7 Ser/Thr residues within the N-terminal transactivation domain 1 (TAD1) play a role in p53 activation, yet their precise mechanisms of action remain elusive due to challenges in accessing well-defined phosphorylated isoforms. To address this limitation, this study harnesses a recently developed approach for the semisynthesis of site-specifically phosphorylated p53 to generate a comprehensive library of singly phosphorylated p53 including all TAD1 sites: Ser6, Ser9, Ser15, Thr18, Ser20, Ser33, and Ser37. The library was then used to probe the specificity of common p53 antibodies in western blot analysis. This study's results confirm the specificity of the target site of most phosphorylation-specific anti-p53 antibodies, but also reveal wide-spread epitope masking by phosphorylation, which has implications for p53 research and diagnostics. This "designer" p53 library thus provides a toolkit to study the function of p53 phosphorylation directly and indirectly as a quality control agent for some of the most widely used reagents in the field.