Introduction. Adult tendon injuries occur very frequently, but injured tendon heals very slowly and the mechanisms of the slow-healing response to injury are still largely unknown. Currently, the main barrier is our insufficient understanding of the mechanisms responsible for homeostasis, regeneration and repair of adult tendon. This gap in knowledge translates to a lack of experimental models. Therefore, using the combination of state-of-the-art genetic approaches, we have established novel cell biological tools to advance the understanding of tendon biology. Materials and Methods. Adult mouse tendon progenitor lines and Adult mouse tenocyte lines: Primary adult tenocytes were isolated from Achilles tendon in Scleraxis(fl/fl)/Scleraxis-GFP/p21(−/−) mice, then CD90.2- and subsequent Sca1-positive cells were sorted by Flow Cytometry. Then Scleraxis-null progenitor lines were generated by the treatment of those cells with adenovirus-Cre. Adult Scleraxis(+/+) and Scleraxis-null tenocyte lines were also generated from Scleraxis(fl/fl)/Scleraxis-GFP/p21(−/−) mice. To establish Scleraxis-Flag overexpressing tenocyte lines, Scleraxis and Flag-tag fusion-protein expression construct was generated and transfected into Scleraxis-null tenocytes (Scleraxis transgenic mouse strains were provided by Dr Ronen Schweitzer). Scleraxis antibody: DNA coding mouse Scleraxis residues were obtained by PCR, then the recombinant protein was expressed, immunized in rabbits, and an affinity-purified antibody was generated. Results. Established parental progenitor lines highly expressed Sca1 (98.9%), CD90.2 (97.3%), and CD44 (99.8%) and were almost negative for ScxGFP (2.3%). Interestingly, Scleraxis-null progenitors showed significantly increased clonogenicity. Furthermore, when stimulated toward mesenchymal lineages, Scleraxis-null progenitors enhanced differentiation into chondrocytes. Our Scleraxis antibody reacted with lysates from cells expressing Scleraxis-Flag fusion proteins (∼30 kDa), whereas it did not react with Scleraxis-null cells by Western analysis. Immunofluorescence analysis of adult mouse Achilles tendons further confirmed intense Scleraxis protein expression in wild-type tenocytes, whereas considerably decreased expression of Scleraxis was evident in Cre-treated Scleraxis(fl/fl) tenocytes. Discussion. These novel tools will be the promising resources to get an insight into molecular framework for Scleraxis in adult tendons. It is anticipated that the establishment of experimental models using these resources will fill major gaps in the current knowledge of adult tendon biology and will facilitate development of novel strategies to treat adult tendon injury