Testis-determining factor (TDF), also known as sex-determining region Y (SRY) protein, is a DNA-binding protein (also known as gene-regulatory protein/transcription factor) encoded by the SRY gene that is responsible for the initiation of male sex determination in humans. SRY is an intronless sex-determining gene on the Y chromosome in therians (placental mammals and marsupials); mutations in this gene lead to a range of sex-related disorders with varying effects on an individual's phenotype and genotype.