PAX3 (paired box gene 3) is a transcription factor that activates and represses target genes that have functional involvement in a wide range of processes including muscular and neural development, neural and melanocyte growth and differentiation, as well as cellular migration, adhesion and apoptosis. Inherited mutations in PAX3 lead to Waardenburg syndrome subtypes WS1 and WS3, described by developmental abnormalities in the eyes, musculoskeletal deformities of the upper limbs, abnormal facial structure, pigmentation disorders and loss of hearing. PAX3 translocation fusions are common in sarcomas such as alveolar rhabdomyosarcomas and biphenotypic sinonasal sarcoma, and these fusion proteins have an increased activation potential for PAX3 target genes. This leads to a consequential upregulation of transcribed proteins that impact rates of cellular proliferation, death and migration potential and contribute to tumorigenesis in the disease. In immunohistochemistry, PAX3 has nuclear positivity in most tissues throughout the body. References: Seminars in Cell & Developmental Biology. 44: 87–96, PMID: 26410165; Human Mutation. 31 (4): 391–406,PMID: 20127975; Cancer Research. 66 (14): 6936–46, PMID: 16849537; Nature Genetics. 46 (7): 666–8, PMID: 24859338;