The key differences in the MMUY and MMU18 characteristics between NCBI Build 37.2 and Build 38.1, in regard to number (#) of contigs, chromosome size (Size), number of gaps on chromosome (Gap), and number of genes (Gene), are summarized. B. The karyotyping images of chromosomes from a male mouse (C57BL/6J strain) were obtained from the Jackson Laboratories (Bar Harbor, ME) website to compare the microscopic sizes of MMUY and MMU18, which are similar in nucleotide length, ~91.7 Mb and ~90.7 Mb, respectively (Akeson and Davisson 2001; Guda et al. 2004). MMUX (~171 Mb) serves as a reference.