Hirokazu Tsukaya obtained his Ph.D. in botany from the University of Tokyo in 1993. After six years as an Assistant Professor in the Institute of Molecular and Cellular Bioscience at the University of Tokyo, he spent the next six years as an Associate Professor in the National Institute for Basic Biology. In October 2005, he returned to the University of Tokyo to become Professor of Evolutionary Biology.   Professor Tsukaya's primary research focus is on the mechanisms controlling leaf shape and size. He initiated these studies by discovering that the two-dimensional growth of leaf blades is controlled by two independent, polarity-dependent systems in a model plant species, Arabidopsis thaliana (L.) Heynh. In each case, polarity-dependent expansion is due to differential regulation of two different mechanisms, namely, control of polar cell expansion and control of polarity-dependent cell cycling. His research team has identified genes involved in all four of the above developmental systems using both a genetic and genomic approach. More recent work includes analyses of mechanisms determining leaf size. He coined the term, 'Compensation' to define abnormal cell enlargement triggered by a significant decrease in the number of cells per leaf primordium, resulting in a leaf of similar size to one that is not perturbed. This curious phenomenon is, he thinks, an important clue to understand organ-level control of cell behavior and the mechanisms of organ size determination via cell-to-cell communication. Presently, working in both the University of Tokyo and in the National Institute for Basic Biology (Okazaki, Japan), he is working on data obtained from analyses of the original mutants of A. thaliana.