A majority of genetically modified mice carry passenger mutations, originating from the 129-derived embryonic stem (ES) cells, near the targeted gene. Unintended retention of these mutations can introduce confounding phenotypes and affect conclusions. We show that Ackr1-/-129ES mice retained approximately 6 Mb of 129 ES cells-derived genomic material in chromosome 1 near Ackr1 in bone marrow derived monocytes (Mono), nucleated erythroblasts (NECs), and polymorphonucleated neutrophils (PMN), notably affecting the expression of PYHIN genes. Overall design: C57BL/J (n = 3, WT CTRL) and ACKR1-deficient mice (n = 3, KO); no technical replicates