Generating Conditional Knockout Mouse Models

All animal work was performed according to institutional regulations for care and use of laboratory animals and approved by the Lower Saxony State Office for Consumer Protection and Food Safety (LAVES; registration numbers: G15/2039, G15/1754 and G11/540). Usp22^flox mice were generated as described previously⁶ by removing lacZ and neomycin resistance loci from the Usp22^{tm1a(KOMP)Wtsi} C57BL6 mouse line generated from embryonic stem cells obtained from the University of California-Davis Knockout Mouse Project Repository^{20 (link)} by FLP-mediated excision^{21 (link)}. Usp22^flox mice were crossed with MMTV-Cre (a kind gift from L. Henninghausen, National Institutes of Health, USA) and Tg(MMTV-ErbB2)NK1Mul/J (The Jackson Laboratory) animals (FVB/N background) to achieve a mammary-specific Usp22 knockout and to promote tumorigenesis, respectively. Moreover, Usp22^flox mice were crossed with Villin-Cre^ERT2 and Apc^1638N mouse lines (C57BL/6N background) to achieve an intestinal knockout and to investigate its role in tumorigenesis⁶ . The Apc^1638N/+ mouse line was a kind gift from F. Bosman (Erasmus University Medical Center Rotterdam, The Netherlands). Multiple replicates (n = 3–6) were utilized in order to ensure reproducibility of findings.