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Abstract-Recently, the side population (SP) phenotype has been introduced as a reliable marker to identify subpopulations of cells with stem/progenitor cell properties in various tissues. We and others have identified SP cells from postmitotic tissues, including adult myocardium, in which they have been suggested to contribute to cellular regeneration following injury. SP cells are identified and characterized by a unique efflux of Hoechst 33342 dye. Key Words: Abcg2 Ⅲ Mdr1 Ⅲ progenitor cells Ⅲ proliferation Ⅲ SP cells R ecently, the side population (SP) phenotype has been introduced as a reliable marker to identify subpopulations of cells with stem/progenitor cell properties in various tissues including the heart. 1 On the molecular level, the SP phenotype is linked to the presence of ATP-binding cassette (ABC) transporters with the ability to efficiently efflux the DNA binding dye Hoechst 33342. 2 This ABC transporterdependent Hoechst efflux phenomenon confers the characteristic fluorescent-activated cell sorting (FACS) profile of SP cells as a Hoechst-low "side population" located to the periphery of the Hoechst-high main population. 2 Among the various members of the ABC transporter superfamily, Abcg2 (also referred to as breast cancer resistance protein 1 [Bcrp1]) and Mdr1 (also referred to as P-glycoprotein [p-gp] or Abcb1) have been shown to efficiently efflux Hoechst 33342 and thereby confer the SP phenotype. 3 Although both transporters are highly expressed in bone marrow (BM)SP cells, studies performed in mice with targeted disruption of the Mdr1a and Mdr1b genes, the murine homologs of the human Abcb1/Mdr1 gene, demonstrated that Abcg2 is the sole molecular determinant of the SP phenotype in hematopoietic stem cells. 4 Moreover, Abcg2 expression is conserved in SP cells from a wide range of tissues including blood, gonad, lung, skeletal muscle and the retina, suggesting an important role of Abcg2 in stem cells. 4 -7 We and others have characterized SP cells isolated from adult myocardium. 8 -11 These cardiac (c)SP cells are phenotypically distinct from BMSP cells, in that they are not hematopoietic but exhibit the potential to differentiate into functional cardiomyocytes. 10 As in SP cells from the bone marrow, Abcg2 is expressed in SP cells from the heart. 9 The contribution of Abcg2 to the cSP phenotype and its biological significance in cSP progenitor cells, however, remain unknown. In this study, we find that the contribution of Abcg2 to the SP phenotype in the heart exists in an age-dependent manner, with Abcg2 as the molecular determinant of the SP phenotype in the neonatal heart and Mdr1 as the basis for the SP phenotype in the adult heart. In addition, we demonstrate Original