Joel made mice with a conditional allele of calcineurin B1, the ubiquitous regulatory component of calcinerin phosphatase complexes. He found that calcineurin was essential for positive but not negative selection of thymocytes. In addition, he worked with Ching-pin Chang in the lab to show that calcineirn and NFATc proteins are essential for a regulatory exchange between myocardial and endocardial cells that resulted in heart valve development. Joel’s alleles of calcineurin are now being used to define the role of calineurin in many aspects of mammalian development, revealing that it may be one of the most widely used signaling pathways in mammalian development. He is presently in Phil Sharp’s lab at MIT, working on the role of small RNAs in lymphocyte development. He can be reached at: jneilson@MIT.EDU.