Leonard Zon, the Grousbeck Professor of Pediatric Medicine at Harvard Medical School, a Howard Hughes Medical Institute Investigator, and Director of the Stem Cell Program at Boston Children's Hospital, is a consummate clinician scientist. Len’s pioneering research has led to fundamental discoveries in hematopoiesis, cancer biology and stem cell biology all while developing, harnessing and showcasing the power of the zebrafish as a model organism.

Following upon an exciting research track record in hematopoiesis as a hematology clinical fellow and postdoc, Len burst into the zebrafish scene as a young PI in 1993 to use this then nascent model organism to understand hematopoietic transcription factors in the context of development. This was the heyday of large-scale zebrafish mutational screens to identify and map mutants in early development, and Len realized the unique potential of the zebrafish to probe hematopoiesis. He performed wide-ranging screens for both red and white blood cell mutants, being among the first five groups to establish zebrafish genomics and maps. His screens identified five hematopoiesis mutants with uncharacterized genes, and went on to find humans with hematopoiesis defects who carried mutations in them, thus discovering new human diseases.

These early discoveries set the stage. Len went on to use the zebrafish to understand the molecular nature of hematopoietic stem cells and their niches, so as to determine how one might enrich them for therapeutics. Now Len’s work is at the cutting edge, providing mechanistic understanding into the aging of hematopoietic stems cells using new technologies such as cellular barcoding; this fascinating biology could well provide sorely needed remedies for yet another bane of transplantation medicine. Len was a major contributor to developing the zebrafish as a model for cancers, and his work in melanoma is paying spectacular dividends, with phase one studies ongoing for a completely new melanoma therapy.

Len is one of the most inventive zebrafish researchers. His work couples his uncanny biological insights and instincts to his spot-on feel for technology. He is a rapid developer, embracer and modifier of technologies, thus benefitting not only his own work but the entire field. Len epitomizes the generosity of the zebrafish field; he freely shares resources, technologies, and tools regardless of whether one is competing with him or not. His infectious enthusiasm coupled with his generosity and sage advice are responsible for many of us starting to use the zebrafish to study other diseases and being successful at it. This influence is on top of directly training 81 individuals, 29 of whom hold faculty positions with others in biotech and other science jobs. Len continues to go strong, with 21 trainees currently. Len may have won every mentoring award possible - the Harvard Postdoc and BBS Graduate Student Mentoring Awards and the American Society of Hematology Mentoring Award.

Len’s community and advocacy work have earned the admiration and gratitude of the zebrafish research community. He pitched the zebrafish genome project to the NIH and became the leader of the Trans NIH zebrafish consortium. He convinced the NIH to set up the zebrafish tools grants. These achievements are all the more amazing coming as they did during a low point in NIH funding. Len has organized multiple zebrafish meetings, and sensing the enthusiasm surrounding the newly initiated annual zebrafish disease models meetings, Len worked with others to start the Zebrafish Disease Models Society. He remains a tireless, enlightened zebrafish advocate through this Society and other venues. Len has recently been elected to the National Academy of Sciences.

Four of Len’s discoveries in the zebrafish have moved to human phase one clinical trials. One of these, using prostaglandin E2 to increase stem cell numbers, is currently in phase two clinical trials to improve stem cell engraftment after myeloablative therapy of leukemia with over 150 patients treated. Len’s discoveries could have life-saving impact.

Len is a brilliant scientist, a wonderful, passionate, compassionate human being whose work is providing biological insights and cures for intractable diseases, who is passing on the baton beautifully to the next generations of zebrafish researchers and who is a terrific advocate for our field.