Germline-mediated ubiquitous recombination in ScxCre male mice: implications for tendon research

Scleraxis (Scx), a basic helix-loop-helix (bHLH) transcription factor, is a primary marker for tendon and ligament lineages. Consequently, mouse models utilizing Cre recombinase under the control of the Scx locus represents a powerful tool for control of gene expression in tendon. The constitutive ScxCre mouse line is widely used for tendon-specific genetic manipulation. In this study, we demonstrate that ScxCre exhibits undesired significant off-target activity in the male germline, leading to ubiquitous recombination of floxed alleles in all tissues of the resulting offspring. This inheritance of recombined LoxP alleles occurs independently of Cre inheritance, indicating that ScxCre-induces recombination occurs prior to meiosis in diploid germ cells. This off-target activity is not observed in female germline. These findings highlight a critical need for stringent parental sex selection when using ScxCre lines to ensure tissue-specific targeting and avoid unintentional global gene deletion or transgene activation.