There is still much to learn about the cells used for cell- and gene-based therapies in the clinical setting. Stem cells are found in
virtually all tissues in the human body. As a result, cells isolated from these tissues are a heterogeneous population consisting of
various subpopulations including stem cells. Several strategies have been used to isolate and define the subpopulations that
constitute these heterogeneous populations, one of which is the side population (SP) assay. SP cells are identified by their ability
to efflux a fluorescent dye at a rate that is greater than the main cell population. This elevated rate of dye efflux has been
attributed to the expression of members of the ATP-binding cassette (ABC) transporter protein family. SP cells have been
identified in various tissues. In this review, we discuss the research to date on SP cells, focussing on SP cells identified in
haematopoietic stem cells, adipose-derived stromal cells, and dental pulp.