For the first time, the presence of a new alternative promoter in the gene of the oct-1 transcription factor from which a previously unknown mRNA isoform Oct-1X, with 5'-terminus different from the previously described isoforms, was demonstrated. The nucleotide sequence of the Oct-1X cDNA was determined and the presence of a long open reading frame which starts with the first ATG codon of the second exon was demonstrated. A protein shortened on its N-terminus is assumed to be the product of a new mRNA isoform. Expression of the new isoform was studied in various human cell lines; the results indicate the ubiquitous nature of the Oct-1X expression. Therefore, in the work, the third alternative promoter of human oct-1 gene (earlier, a ubiquitous promoter U and an inducible tissue-specific promoter L were described) contributing to the fine regulation of mRNA isoform synthesis and formation of structural and functional diversity of Oct-1 protein isoforms in cells was revealed.