Lo¨ ser, Schirm, Guhr et al. question whether the nearly 700 cell lines reported in the sci- entiﬁc literature have all the characteristics of human pluri- potent cell lines and are available for research. For example, characterization data in scientiﬁc articles are often only pre- sented exemplarily for individual cell lines, whereas deriva- tion of multiple cell lines was reported. Furthermore, more than 400 hESC lines were described or used in only a single study with no follow-up studies so far. On the other hand, our list contains several as yet unpublished hESC lines with characteristics of true human pluripotent stem cells. For example, there are 14 hESC lines that were not published in the scientiﬁc literature until the end of 2008, but were accepted by the United Kingdom stem cell bank, which requires a speciﬁc level of characterization. Similarly, 68 hESC lines derived at institutions in the United States and not reported in the literature so far have been announced for NIH registration between September and November 2009. It is most likely that these cell lines will also have characteris- tics of truly pluripotent cells. Moreover, several companies, such as Cellartis, Novocell, or Advanced Cell Technology, Inc. with extensive experience in hESC research have reported the existence of hESC lines for which no characteri- zation data have yet been published in the scientiﬁc litera- ture. Some cell lines from Russia and China have obviously been well-characterized but only published in Russian or Chinese language journals, respectively. Therefore, it remains difﬁcult to determine the number of well-characterized and available hESC lines. While the International Stem Cell Initi- ative characterized 59 hESC lines from 17 countries and was able to derive common and robust molecular patterns as a step toward the standardization of these cells [3], the actual level of characterization for most hESC lines is highly vari- able, often incomplete in accessible registries, and thus of limited value for comparison. We also analyzed the extent and impact of international research involving hESCs. By 2008, the number of hESC research papers had more than tripled compared to the pe- riod from 1998 to 2005 [2]. In the same period, the average impact factor of journals in which original hESC work was published increased from 6.03 to 7.422. Thus, the high aver- age impact factor of hESC research is probably not a mere consequence of the novelty of the hESC ﬁeld but rather reﬂects a continuous and spreading interest in this type of research. Our data also underscore the leading position of the United States in hESC research with respect to quantity and quality of this research, which remained unchanged over the last decade. Other studies found an increasing productiv- ity gap in US hESC research and an underperformance of US research in the ﬁeld, respectively [16, 17]. The reasons for the differing assessments may be partially due to a dif- ferent data pool used. For example, Levine [17] included in his analysis all journal articles citing the ﬁrst hESC paper by Thomson et al. [15] and therefore might have included articles that do not report original hESC work but rather summaries of or reﬂections on work with hESC cells. On the other hand, many more recent hESC research papers do not cite Thomson et al. [8]. In contrast, we have extensively analyzed all articles extracted by our analysis and even excluded papers describing previously reported work in articles published in rather methodical journals such as Na- ture Protocols, Stem Cell Protocols, or Methods in Molecu- lar Biology. Our data further reveal the preferential use of only a few cell lines in international hESC research until the end of 2008. We found a pronounced predominance of cell lines pro- vided by the WiCell Research Institute, which was even stronger in US-based studies. However, our data also uncover www.StemCells.com 245 regional differences in the use of hESC lines. The reasons for this phenomenon are not clear, and explanations remain speculative. For example, the low usage of WiCell lines in Australia, Korea, or Sweden might be due to the fact that these countries had their own hESC lines available at the start of international hESC research in the early years of this decade. On the other hand, the high percentage in use of WiCell lines in work from Great Britain and Canada may be explained by the early need of cells for hESC work in these countries before the later availability of domestic stem cell lines. The preferential use of WiCell lines in hESC studies from Israel (64.8% of research papers despite the early avail- ability of their own hESC lines) might be due to the involve- ment of Israeli scientists in the derivation of the ﬁrst WiCell lines and the establishment of well-suited sublines of H9 in Israel. However, a considerable portion of studies from Israel did not contain information on the speciﬁc cell lines used. Importantly, we did not ﬁnd proof for a direct correlation between the nature of a country’s stem cell research policy (restrictive versus permissive) and the use of certain hESC lines nor for an indirect transmission of US stem cell policies to other regions due to highly collaborative research. Thus, other factors, such as early publicity by the provider or by early users, distribution policy, or the grade of characteriza- tion and documentation of a cell line (which increases with the frequency of its use), might contribute to the decision for using a speciﬁc cell line. While this manuscript was in preparation, Scott et al. published a study on the use of human embryonic stem cell lines [18]. The authors found a strong predominance of only two hESC lines, WiCell H1 and H9, in experimental hESC research. This ﬁnding was based on the number of requests for certain hESC lines to the NIH Stem Cell Bank and to the HSCI, and on the evaluation of 534 publications for which hESC lines were used. Unfortunately, it is not obvious from this study whether it was focused on the US situation only or also included hESC work from other countries. Moreover, it is not clear what literature data pool was used for this analysis. Although our data, in prin- ciple, conﬁrm the ﬁnding on the predominance of WiCell lines, there are also major differences to those reported by Scott et al. For example, these authors found that 83.3% and 60.9% of all hESC studies were based on the H9 and H1 cell lines, respectively, whereas we found evidence for application of these cell lines and their derivatives in 36.7% and 30.8% of hESC studies worldwide, respectively. This discrepancy is most likely due to a more comprehen- sive data set used in our study. Whether the US stem cell policies and resulting NIH funding restrictions are responsi- ble for the preferred use of just two WiCell lines still remains an open question. In summary, our data provide a detailed picture on the status of international hESC research based on a systematic analysis of the entire peer-reviewed literature. The persistent dominance of the WiCell lines H1 and H9 in research also points to a potential weakness with respect to universal applicability of the results, standardization, and harmoniza- tion in the ﬁeld. The collection and dissemination of a wide range of data, including negative data, for as many hESC lines as possible (for example, by international registries such as hESCreg, the European Human Embryonic Stem Cell Registry) may be one option for the diversiﬁcation of hESC use. International registries may then provide the answer to researchers as to whether it makes a difference which cell line to use, or to choose the best cell line for their research or application.