What's in a (Sub)strain?

C57BL/6J and C57BL/6N inbred mice are widely, and often interchangeably, used for stem cell research; yet, these substrains harbor discrete genetic differences that can cause phenotypic disparities. In this issue of Stem Cell Reports, Morales-Hernandez et al. identify one particular difference-disruption of Nicotinamide Nucleotide Transhydrogenase (Nnt)-that increases reactive oxygen exposure and impairs hematopoietic progenitor cell function in C57BL/6J, as compared to C57BL/6N, mice.