Complex sentence: Mutations that abrogate Janus kinase 3 function cause an autosomal SCID (severe combined immunodeficiency disease), while activating Janus kinase 3 mutations lead to the development of leukemia.
Split into simpler sentences: Mutations that abrogate Janus kinase 3 function cause an autosomal SCID (severe combined immunodeficiency disease). Activating Janus kinase 3 mutations lead to the development of leukemia.