Apoptosis is the process of programmed cell death, which occurs as a normal physiologic process and plays an important role in embryonic development and maintenance of tissue homeostasis. It is characterized by special morphologic features, including asymmetric loss of plasma membrane and attachment- as one of the earliest features, condensation of the nuclear chromatin, internucleosomal cleavage of DNA and cell membrane blebbing. During apoptosis, the membrane phospholipid phosphatidylserines (PS) are translocated from the inner to the outer layer of the plasma membrane and exposed to the external cellular environment.

Annexins are a family of calcium-dependent phospholipid-binding proteins that can bind to PS. Fluorophoe-conjugated Annexin V is commonly used to visualize apoptosis via detection of externalized phosphatidylserine residues on the outer layer of plasma membrane in apoptotic cells. Binding of Annexin V and PS occurs in the presence of Calcium ion (Ca²⁺). Upon the attachment of apoptotic cells to labelled Annexin V, apoptosis can be measured using fluorescent microscopy or flow-cytometry.