Pioneer transcription factors constitute a heterogeneous group of regulatory animal proteins that, unlike other transcription factors, are able to recognize and bind target DNA sequences within condensed chromatin. This binding can alter the local chromatin structure and facilitate the binding of other proteins, thus contributing to subsequent events required for gene regulation. The ability to bind to the silent genes in a closed environment makes pioneer factors an extremely important tool in such processes, which that are accompanied by fundamental restructuring of the cell phenotype as differentiation during embryonic development and cell reprogramming. These proteins are able to remain bound to the target sequences during mitotic division and are likely to participate in the maintenance of cellular memory. Pioneer factors are likely to take an active part in the processes of carcinogenesis and maintaining the tumor cell phenotype, but their role in these processes requires additional examination. It is reasonable to suppose that further study of transcription factors of this group will make it possible to better understand the genetic processes in embryonic development, to improve the efficiency of cell reprogramming, as well as to develop new approaches to cancer diagnostics and therapy.