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It typically contains a 3\u2032 poly(A) tail required for optimal stabilization and translation of the mRNA. The average length of the poly(A) tail in the produced mRNA cannot be precisely predicted and consequently must be measured using complicated technologies such as reverse transcription and sequencing or cleavage of the poly(A) tail and analysis by capillary electrophoresis or mass spectrometry. We report an accelerated method to evaluate the average poly(A) tail length in synthetic mRNA, which benefits from the fact that thiazole orange is fluorescent only when in close proximity of nucleic acid. An oligo(dT)12 oligonucleotide having thiazole orange at its extremities emits a fluorescence signal proportional to the amount of poly(A) sequence available. Using a titration curve made with known amounts of poly(A) RNA oligonucleotide, the thiazole orange oligo(dT)12 oligonucleotide can instantly indicate the average length of the poly(A) tail in an mRNA sample. This fast and easy method can be used in any laboratory to determine the size of the poly(A) tail in in vitro-transcribed mRNA produced for research, pre-clinical studies and clinical trials.<\/jats:p>","DOI":"10.3390\/mps9040109","type":"journal-article","created":{"date-parts":[[2026,7,13]],"date-time":"2026-07-13T09:51:09Z","timestamp":1783936269000},"page":"109","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["A Simple Method to Evaluate the Length of Poly(A) Tails in mRNA"],"prefix":"10.3390","volume":"9","author":[{"ORCID":"https:\/\/orcid.org\/0009-0007-6512-4716","authenticated-orcid":false,"given":"Jonas","family":"Mumenthaler","sequence":"first","affiliation":[{"name":"Department of Dermatology, University Hospital Z\u00fcrich (USZ), University of Z\u00fcrich (UZH), 8091 Z\u00fcrich, Switzerland"},{"name":"Faculty of Medicine, University of Z\u00fcrich, 8091 Z\u00fcrich, 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