Pioneer transcription factors are essential for enabling gene expression by accessing DNA in condensed chromatin, where most DNA is occluded by nucleosomes and higher-order chromatin structures. These factors can bind to DNA sites before other transcription factors, initiating regulatory events. They can act passively by reducing the number of factors needed for binding or actively by opening chromatin to allow other factors to access it. Examples include FoxA and GATA factors, which are crucial in embryonic development, hormone induction, and cancer. Pioneer factors like FoxA1 and GATA3 are involved in liver development and can reprogram cells to hepatocyte-like states. They also play roles in hormone-responsive cancers, such as breast and prostate cancer, where they facilitate the binding of estrogen and androgen receptors to chromatin. Chromatin modifications, histone variants, and DNA methylation influence the accessibility of target sites, and pioneer factors can modulate these to enable transcription. In cancer, the loss of pioneer factors can lead to inappropriate gene activation, highlighting their importance in maintaining chromatin competence. Targeting pioneer factors may offer new therapeutic strategies for hormone-driven cancers. Overall, pioneer factors are critical for establishing transcriptional competence in cells, enabling the regulation of gene expression in development, stem cells, and disease.Pioneer transcription factors are essential for enabling gene expression by accessing DNA in condensed chromatin, where most DNA is occluded by nucleosomes and higher-order chromatin structures. These factors can bind to DNA sites before other transcription factors, initiating regulatory events. They can act passively by reducing the number of factors needed for binding or actively by opening chromatin to allow other factors to access it. Examples include FoxA and GATA factors, which are crucial in embryonic development, hormone induction, and cancer. Pioneer factors like FoxA1 and GATA3 are involved in liver development and can reprogram cells to hepatocyte-like states. They also play roles in hormone-responsive cancers, such as breast and prostate cancer, where they facilitate the binding of estrogen and androgen receptors to chromatin. Chromatin modifications, histone variants, and DNA methylation influence the accessibility of target sites, and pioneer factors can modulate these to enable transcription. In cancer, the loss of pioneer factors can lead to inappropriate gene activation, highlighting their importance in maintaining chromatin competence. Targeting pioneer factors may offer new therapeutic strategies for hormone-driven cancers. Overall, pioneer factors are critical for establishing transcriptional competence in cells, enabling the regulation of gene expression in development, stem cells, and disease.