A distinct "side population" of cells in human tumor cells: implications for tumor biology and therapy

Cell Cycle. 2005 Feb;4(2):203-5. Epub 2005 Mar 1.


Stem cells have an extensive capacity to proliferate, differentiate and self-renew. In many mammals, including humans, an adult stem cell subpopulation termed the "side population" (SP) has been identified. SP cells can rapidly efflux lipophilic fluorescent dyes to produce a characteristic profile based on fluorescence-activated flow cytometric analysis. Previous studies have demonstrated SP cells in bone marrow obtained from patients with acute myeloid leukemia, suggesting that these cells might be candidate leukemic stem cells, and recent studies have found a SP of tumor progenitor cells in human solid tumors. These new data indicate that the ability of malignant SP cells to expel anticancer drugs may directly improve their survival and sustain their clonogenicity during exposure to cytostatic drugs, allowing disease recurrence when therapy is withdrawn. Identification of a tumor progenitor population with intrinsic mechanisms for cytostatic drug resistance might also provide clues for improved therapeutic intervention.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • ATP-Binding Cassette Transporters / genetics
  • ATP-Binding Cassette Transporters / physiology
  • Antineoplastic Agents / pharmacology
  • Antineoplastic Agents / therapeutic use
  • Cell Proliferation
  • Cell Survival
  • Drug Resistance, Neoplasm
  • Flow Cytometry
  • Gene Expression Regulation, Neoplastic
  • Humans
  • Neoplasms / drug therapy
  • Neoplasms / genetics
  • Neoplasms / pathology*
  • Neoplastic Stem Cells / drug effects
  • Neoplastic Stem Cells / pathology*
  • Neoplastic Stem Cells / physiology


  • ATP-Binding Cassette Transporters
  • Antineoplastic Agents