Many researchers are not aware of the critical importance of genetic background of mice, including the concept of inbred substrains, and how the genetic background of substrains can impact their research. There are many different substrains of C57BL/6 mice and this presentation will not only define and explain the development of these substrains, but will also provide scientific data demonstrating how they differ in both genetics and phenotype. Participants will learn the importance of understanding what C57BL/6 substrain they are working with, how to determine what substrain they have, and use this information to ensure they are using the appropriate wild type controls to properly interpret their scientific results.