When I became a physician and an endocrinologist in the early 1960s, peptide hormone sequencing was still in its infancy; it was also far removed from my immediate interests. Through chance encounters with prominent teachers and mentors, I later became increasingly convinced that elucidation of the primary sequence of peptide hormones is key to understanding their production as well as their functions in human health and disease. My interest for pituitary hormones led me to discover that the sequence of β-melanocyte-stimulating hormone was contained within that γ and β-lipotropins and could be released from the latter by limited endoproteolysis. This prohormone theory became the leitmotiv of my career as a clinician/scientist. Through serendipity and the efforts of many laboratories including mine, this theory has now been widely confirmed, extended to various precursor proteins and implicated in many diseases. It has led to our discovery of the proprotein convertases.