A fundamental understanding of all biological processes requires an understanding of the molecular basis of cellular function. The discipline of molecular biology focuses on the genetic information in cells: how is the inherited information encoded within DNA and how is this information regulated and expressed so that the cells of a multicellular organism develop from a single cell to highly specialized cells in a complex and integrated organism? For the practicing clinician, an understanding of the molecular mechanisms by which a differentiated cell develops and maintains its specialized functions is critical to a more in-depth understanding of human diseases. This is the 1st of a 3 part series on molecular biology for the clinician. This article introduces general principles of nucleic acid and gene structure. The 2nd article will examine commonly used research and diagnostic techniques based on the principles of molecular biology. The 3rd part will examine the frontiers of gene therapy, a novel approach to the treatment human diseases at the molecular level.