Likewise, tumor initiating cells are a subset of cells within a tumor cell population, which, also through asymmetric division, retain the ability to reconstitute tumors. The side population phenotype was first described in bone marrow, where a somatic stem cell population was identified based on its ability to extrude the DNA binding dye Hoechst 33342, a AZ 960 JAK inhibitor phenomenon that is associated with the expression of ATP-binding cassette transporter G2. In general, the SP phenotype is thought to be a universal marker of somatic stem cells and has been used to isolate them from many adult tissues, such as the myometrium, endometrium and mammary gland. Leiomyomas are thought to be monoclonal tumors arising from the myometrium; however, it is not known what cell population in the myometrium gives rise to these tumors. Several recurrent genetic aberrations, such as trisomy of chromosome 12, deletions in 7q, and mutation affecting the mediator complex subunit 12 or the high mobility group AT-hook 2 gene were reported in uterine leiomyomas. As in other diseases, these genetic abnormalities and tumor stem cells are considered to play pivotal roles in the tumorigenesis of leiomyoma. To investigate the possible role of stem cells in human uterine leiomyoma growth, we examined leiomyoma derived SP cells. Uterine leiomyomas are monoclonal tumors, with growth of the neoplasm occurring via clonal expansion from a single cell; this raises the possibility for development of new, targeted therapeutic interventions. Our results reinforce the hypothesis that LMSP are likely involved in leiomyoma tumorigenesis. Most leiomyomas contain specific genetic mutations suggesting that transformation of normal myocytes into abnormal myocytes is required at some point during the genesis of a leiomyoma. This process appears to be quite common, in view of the high prevalence of microscopic leiomyomas. We believe that LMSP arise from this myometrial transformation, though the timing of this transformation has not been elucidated and may occur in adults or during an embryonic stage. The myometrium itself has a prominent regenerative capacity. As uterine leiomyomas are thought to be derivatives of myometrial cells, we hypothesize that a population of putative stem/ reservoir cells that support growth exists within the leiomyoma. In support of this hypothesis, the Hoechst-stained cells contained a small fraction of SP cells that were resting in G0 phase, a characteristic of stem cells that allows them to persist in various tissues. Real-time RT-PCR data revealed that the LMSP rarely expressed steroid hormone receptors and smooth muscle cell markers. However, after culture, these markers were expressed naturally at the same levels as seen in LMMP and the total leiomyoma fraction. These results indicate that LMSP represent a population of cells that exist in an undifferentiated state within the leiomyoma that have the potential to differentiate into uterine leiomyoma cells within the environment of the uterine myometrium. Steroid hormones play a role in the growth of uterine leiomyomas and endometrium. Upregulation of ESR1, PGR and aromatase in leiomyoma tissue were reported. The enzyme aromatase, which is encoded by the CYP19A1 gene produces estrogens, appears to be an important regulator of estrogen response in leiomyomas.