Lo¨ ser, Schirm, Guhr et al. 243 Figure 2. Use of hESC lines of selected providers in international research and in published research from selected countries. A: The share of cell lines from a speciﬁc provider in the cell lines used overall in hESC research papers published 1998–2008. BresaGen, BresaGen, Inc., Athens, Georgia; Cellartis, Cellartis AB, Go¨teburg, Sweden; ESI, ES Cell International Pte Ltd, Singapore; HSCI, Harvard Stem Cell Institute, Massachu- setts; Technion, Israel Institute of Technology, Haifa, Israel; UCSF, University of California San Francisco, California; WiCell, WiCell Research Institute, Madison, Wisconsin. Cell lines from other providers contain hESC lines from 48 providers such as Karolinska Institute, Stockholm, Sweden (3.4%); Seoul National University, Seoul, Korea (3.0%); Kyoto University, Japan (2.4%); University of Newcastle, United Kingdom (1.8%); and University of Shefﬁeld, United Kingdom (1.3%). Note that in many studies more than one hESC line was used. B: The percentage of hESC papers from a speciﬁc country produced with cell lines of the given providers. Cell lines from providers other than those speciﬁcally named were grouped in ‘‘other providers.’’ Canadian groups were cited more often (averages of 19.5 and 17.4 citations in 2008, respectively), while work from Chi- nese, Korean, and Swedish groups were cited less frequently (6.7, 7.1, and 8.1 times in 2008, respectively). Moreover, among the 20 hESC research papers cited most frequently in the scientiﬁc literature in 2008, ﬁfteen (75%) were from groups in the United States, and US groups were involved in two others (supporting information). These ﬁndings extend and conﬁrm our own earlier results on the leadership of US research in the hESC ﬁeld [2, 8]. hiPSCs show many, if not all, characteristics of hESCs, but production of hiPSC lines does not require the destruction of human embryos. Additionally, potential clinical use of au- tologous hiPSCs might not be hindered by immunological problems inherent to allogeneic hESC-derived cells. Since the ﬁrst description of hiPSCs in 2007, it was speculated that research using hiPSCs may replace hESC research in the near term, and that hESCs would only be needed for comparison in the future. Some researchers involved in cloning and hESC research even claimed to concentrate solely on hiPSC research in the future [14]. Therefore, we considered to what extent research involving hiPSCs and hESCs would overlap and whether there were indications that hESC research would decline. In a ﬁrst step toward investigating this question, we analyzed the number of papers describing experimental work involving hiPSCs by applying the same criteria as for experi- mental hESC studies and using the search string described in the supporting information. Of 16 hiPSC research papers extracted by our search of 2008 publications, 14 (87.5%) also www.StemCells.com reported application of hESCs or hESC-derived materials. In 2009, the number of experimental hiPSC studies published in English-language peer-reviewed journals dramatically increased to 66 (preliminary data for January 1 to September 30, 2009). Of these studies, 47 (71.2%) also involved applica- tion of hESCs or hESC-derived materials. The number of ex- perimental hESC studies for the same period was 342 (prelim- inary data for January 1 to September 30, 2009) showing a further expansion of international hESC research in 2009. These ﬁndings allow for the conclusion that, at least in 2008 and 2009, both research ﬁelds existed independently, were further extended, and partially overlap. Moreover, the vast majority of studies on hESCs published in 2009 did not involve hiPSCs but, conversely, research on hiPSCs partially involved the application of hESCs or hESC-derived materials. Use of hESC Lines in International Research Finally, we were interested to determine the use of hESC lines in international research. A complete and detailed list on the hESC lines used in scientiﬁc studies investigated here is pro- vided as supporting information. Of 989 papers, 964 contained information on the speciﬁc hESC lines used. As shown in Fig- ure 2A, most international hESC research published between 1998 and 2008 was performed with hESC lines provided by WiCell, which were derived in 1998 by Thomson and col- leagues [15], or derivatives thereof. In 53.8% of all hESC studies published by end of 2008, at least one WiCell line or a derivative of such a line was used either without (36.3%) or with (17.5%) cell lines from other producers. Cell lines from