Since July 1988 I have been an Assistant Professor in the Department of Anatomy and Cellular Biology, with a laboratory on the quadrangle of Harvard Medical School. In July 1993 I will join the new Department of Cell Biology and Physiology, which is intended to have a stronger presence in modern cell and developmental biology. My long term goal is a career in academic research and graduate education. For the immediate future my research interests will focus on the biology and biochemistry of DNA modification in mammals, which has recently been implicated as a factor in imprinting diseases, mutation, and ectopic gene inactivation. I was the first to purify a eukaryotic DNA methyltransferase to homogeneity and to clone and sequence the cDNA for such an enzyme. We recently succeeded in constructing strains of mouse which bear partial and severe loss of function mutations in the DNA methyltransferase gene and have shown that even modest reductions (to one-third of normal) in m5C contents have no effect on the growth of embryonic stem cells but cause severe developmental defects and death in embryos. The importance of DNA methylation in mammalian development has long been an uncertain and controversial subject, and possession of the mutant mice will allow rigorous tests of the role of DNA methylation in a variety of biological processes. This proposal focuses on the role of DNA modification in genomic imprinting and X inactivation, the development and application of a versatile probe for the identification of any previously undetected species of DNA methyltransferase (which are undocumented but whose existence is widely suspected), development of a selective inhibitor of DNA methylation through a novel dominant-negative approach, and the biological effects of the minimal and maximal levels of DNA methyltransferase compatible with mouse development. My laboratory now consists of 3 postdoctoral fellows and a graduate student, with a fourth postdoc arriving in September. I bear a very heavy teaching load which this year required more than 200 classroom hours, including over 100 hours of anatomy and histology for medical students. I am also a director of a course for the graduate students entitled """"""""Critical Thought in Cell and Developmental Biology"""""""", and for the last three years have had sole responsibility for a year-long weekly seminar series for the students. The histology and anatomy teaching is onerous because of the time requirement and my lack of training and experience in these disciplines; it is also concentrated in the important beginning of the academic year. An RCDA would be a great help to my research progress as it would allow me to focus on research and graduate education by exempting me from the need to teach in the medical curriculum.