This book is about the role of calcium and calmodulin in the cell nucleus. Calcium, which is an important second messenger of signal transduction pathways, can also operate in the cell nucleus. Different calcium binding proteins, which are the targets of cellular calcium, have been identified in the nucleus of many different cell types. Prominent among these calcium binding proteins is calmodulin, which appears to be involved in the regulation of major nuclear functions such as gene expression and DNA replication.