These data suggest that PIN1 could act as a determinant of cell identity by controlling the activity and expression programs of master TFs that define differentiation toward specific cellular lineages in normal cells (e.g., MEF2C for muscle cell identity, see section PIN1 Controls Stem Cell Reprogramming and Fate), while accumulating data argue for a potential role of PIN1 in supporting cancers that appear to be addicted to dysregulated transcriptional programs and elevated transcription rates, such as leukeamias and breast cancers (102). The gene discussed is PIN1; the disease is breast cancer.