Science in China Series C: Life Sciences © 2007 SCIENCE IN CHINA PRESS Springer www.scichina.com www.springerlink.com Sci China Ser C-Life Sci | Dec. 2007 | vol. 50 | no. 6 | 762-765 Murine fertilized ovum, blastomere and morula cells lacking SP phenotype XU YiXin 1** , HE ZhiYing 2** , ZHU HaiYing 2† , CHEN XueSong 2 , LI JianXiu 2 , ZHANG HongXia 2 , PAN XingHua 3 & HU YiPing 2† 1 ChangZheng Hospital, affiliated to Second Military Medical University, Shanghai 200003, China; 2 Department of Cell Biology, Second Military Medical University, Shanghai 200433, China; 3 Department of Genetics, Yale University School of Medicine, New Haven, CT 06519, USA In the field of stem cell research, SP (side population) phenotype is used to define the property that cells maintain a high efflux capability for some fluorescent dye, such as Hoechst 33342. Recently, many researches proposed that SP phenotype is a phenotype shared by some stem cells and some pro- genitor cells, and that SP phenotype is regarded as a candidate purification marker for stem cells. In this research, murine fertilized ova (including conjugate and single nucleus fertilized ova), 2-cell stage and 8-cell stage blastomeres, morulas and blastocysts were isolated and directly stained by Hoechst 33342 dye. The results show that fertilized ovum, blastomere and morula cells do not demonstrate any ability to efflux the dye. However, the inner cell mass (ICM) cells of blastocyst exhibit SP phenotype, which is consistent with the result of embryonic stem cells (ESCs) in vitro. These results indicate that the SP phenotype of ICM-derived ESCs is an intrinsic property and independent of the culture condition in vitro, and that SP phenotype is one of the characteristics of at least some pluripotent stem cells, but is not shared by totipotent stem cells. In addition, the result that the SP phenotype of ICM cells disap- peared when the inhibitor verapamil was added into medium implies that the SP phenotype is directly associated with ABCG2. These results suggest that not all the stem cells demonstrate SP phenotype, and that SP phenotype might act as a purification marker for partial stem cells such as some pluripo- tent embryonic stem cells and multipotent adult stem cells, but not for all stem cells exampled by the totipotent stem cells in the very early stage of mouse embryos. ABCG2/BCRP, stem cell, fertilized ovum, blastocyst, inner cell mass, embryonic stem cell, SP phenotype It was demonstrated that mouse hematopoietic stem cells with long-term multi-lineage reconstitution abilities could be isolated as a side population (SP) based on their unique ability to efflux the DNA-binding dye Hoechst 33342 [1] . Afterwards, SP cells have also been identified in the hematopoietic compartments of differ- ent species [2] and described in stem cells from other va- riant solid tissues, including the skeletal muscle, lung, liver, heart, testis, kidney, skin, brain, and mammary gland [3] . These findings suggest that the SP phenotype may be a common property of tissue-specific stem cells from adults. Zhou et al. found that the ATP-binding cassette transporter, ABCG2 (termed as MXR/BCRP/ ABCG2), a member of the multiple drug resistance (MDR) family of membrane transporters, is a molecular determinant of this SP phenotype in murine hematopoi- etic stem cells [4] . Other studies in a wide range of tissues have also indicated that the SP phenotype is largely de- termined by the expression of ABCG2 [5] . In all, these previous researches suggest that SP phenotype can be used as a purification marker for isolating potential Received December 31, 2006; accepted August 8, 2007; published online October 4, 2007 doi: 10.1007/s11427-007-0097-y ** These authors contributed equally to this work † Corresponding author (email: zinnia69@gmail.com) Supported by the Shanghai Natural Science Foundation of China (Grant No. 05ZR14147)