Distribution of Germ Cell Competent C57BL/6N ES Cell Lines   Genetic modification by transgenesis and gene targeting has become a well-established method to generate mouse models for the study of gene functions. Functional annotation of these models is one of the biggest challenges in the current biomedical sciences. Accurate phenotyping of these models in the uniform genetic background is essential for this purpose. The C57BL/6N strain is one of the major C57BL/6 substrains. In recent years, efforts have been made worldwide to generate knockout mouse resources of the C57BL/6N background.   The Cell Engineering Division of RIKEN BRC distributes the following germ cell competent C57BL/6N ES cell lines suitable for generating knockout mice. Please contact cellqa@brc.riken.jp to obtain these useful resources. Table 1. Germ cell competent C57BL/6N ES cell lines available from RIKEN BRC Cell Name Cell No. Strain of origin Karyotype Germ cell competency BRC5 AES0009 C57BL/6CrSlc1 2n = 38+XY Confirmed2 BRC6 AES0010 C57BL/6CrSlc1 2n = 38+XX Confirmed2 B6N-22UTR AES0144 C57BL/6NCrlCrlj 2n = 38+XY Confirmed3,4 1C57BL/6CrSlc strain is one of the C57BL/6N substrains   The C57BL/6CrSlc strain is commercially available from Japan SLC, Inc. (Hamamatsu, Japan), and is a C57BL/6 substrain widely used by Japanese researchers. According to the historical records of Japan SLC, the mice were initially received with the strain name of “C57BL/6Cr” by the Institute of Medical Science, The University of Tokyo in 1972 from Mr. Samuel M. Poiley of the National Cancer Institute of the NIH, Bethesda, USA before being transferred to Japan SLC in 1975. Therefore, according to the international rules regarding nomenclature of mouse strains, the C57BL/6CrSlc strain should be designated as C57BL/6NSlc. The N for NIH was missing in the strain name and Cr was used instead, probably as an abbreviation of Cancer research. Our recent genetic analysis using comprehensive SNP markers revealed that the SNP genotypes of the C57BL/6CrSlc mice are identical to those of other C57BL/6N substrains (Mekada et al., Exp Anim 58, 141-149, 2009). Thus, the C57BL/6CrSlc strain is confirmed genetically to be one of the C57BL/6N substrains. Since the substrain name of the C57BL/6CrSlc has widely been recognized among Japanese researchers and used in a number of publications, the nomenclature change should carefully be discussed among users of the mouse. 2Personal communication by Dr. Atsuo Ogura, RIKEN BRC. 3Tanimoto Y, Iijima S, Hasegawa Y, Suzuki Y, Daitoku Y, Mizuno S, Ishige T, Kudo T, Takahashi S, Kunita S, Sugiyama F, Yagami K. Embryonic stem cells derived from C57BL/6J and C57BL/6N mice. Comp Med 58(4):347-352, 2008. 4Recently, knockout mice were successfully generated by using this ES cell line (personal communication by Dr. F. Sugiyama, University of Tsukuba).