This and the report on prostaglandins were designated Citation Classics in 1987. Dr. Klein's research has focused on the enzyme responsible for serotonin acetylation, arylalkylamine N-acetyltransferase (AANAT), and how it is regulated. His functional anatomical studies with several collaborators described the neural circuit responsible for driving circadian rhythms in the pineal gland and established the suprachiasmatic nucleus as the site of the endogenous clock. A highlight of the Section's research was the cloning of AANAT in 1995, which broke a log jam in the field. One of the most important downstream events enabled by this discovery was the crystallization of the AANAT/14-3-3 regulatory complex, which was achieved with NIDDK collaborators in 2001. This advance is noteworthy as the only 14-3-3 complex crystallized with a full length client. Research in the Section on Neuroendocrinology has most recently been dedicated to the goal of describing in genetic and epigenetic terms the regulatory cascades that control cell fate of the pinealocyte, the melatonin producing cell of the pineal gland; and, to the cascades that control daily changes in the expression of thousands of genes in this tissue, work which now heavily relies on high throughput RNA and DNA sequencing. Dr. Klein's honors include the Gregor Mendel Medal by the Czech Academy of Sciences (1995), the Aschoff/Pittendrigh Lecture by the Society for Research on Biological Rhythms (2004), the degree of Doctor Medicinae honoris causa by Johann Wolfgang Goethe University (2005) and the Lerner Frog by the Gordon Research Conference on Pineal Cell Biology (2010).