## 4 Discussion  
Traditionally, methods for the treatment of damage in muscle tissues include the autografting of patient cells or allografting of donor cells [56; 57]. Recently, to overcome limitations derived from the conventional therapeutic approaches, such as stable cell supply problems, multipotential stem-cell-based methods have been studied [10; 12]. However, such improved methods still have problems in cost and supply issues, in terms of using stem cells. As an alternative to solve this problem, the utilization of directly reprogrammed muscle cells from other somatic cells has been spotlighted [35; 36; 58]. According to [PERSON] et al., in 2017, researchers converted human dermal fibroblasts (HDFs) into myogenic lineage by co-transduction of Mycl and MyoD1 through retrovirus. In 2019, [PERSON] et al. converted mouse fibroblasts into skeletal-muscle-like cells through the addition of six small molecules (val/proic acid, Chir99021, repsox, forskolin, parnate, and TTNPB) for 9 days. In 2021, [PERSON] et al. converted mouse fibroblasts into induced myogenic progenitor cells. The mouse fibroblasts were isolated from the transgenic mice, in which _Pax7_ was overexpressed. Then, the isolated mouse embryonic fibroblasts (MEFs) were transduced by lentivirusirus, delivering the MyoD gene. After selection of the efficiently transduced MEFs using antibiotics and fluorescence-activated cell sorter (FACS), cells were treated with small molecules (adenylate cyclase activator forskolin, TGF-\(\beta\) receptor inhibitor repsox, and GSK3-\(\beta\) inhibitor Chir99021). Through three steps over 10 days for direct conversion, MEFs showed induced myogenic progenitor cell-like properties. According to these related former studies, two or more factors were needed to generate directly induced myogenic cells from fibroblasts. In addition, most of the studies dealt with human and mouse fibroblasts, contrary to our research handling bovine fibroblasts and bovine MyoD protein and genes. To promptly generate reprogrammed muscle cells, selection and successful delivery of appropriate transcription factors to somatic cells have been considered to be significant [59; 60]. According to recent studies, efficient delivery methods of transcription factors include the most widely used virus-based methods and the recently used cell-permeable protein-based methods. Although the virus-based methods have the advantage of high efficiency, the risk of transgenes has been constantly raised. On the other hand, the methods using cell-penetrating proteins have been suggested to overcome the safety issues, but their efficiency still needs to be improved. In related studies, MyoD has been studied as an important factor for the direct conversion of human fibroblasts to myoblasts and myotubes [61; 62; 63]. According to [PERSON] et al., MyoD, Myf5, MyoG, and MRF4 have critical roles in skeletal muscle development and these myogenic regulatory factors are responsible for coordinating muscle-specific gene expression in the development of embryos. In this study, we tried to extend the existing results of direct conversion, which are limited to only mouse and human cells, to the results, including cells of domestic animals, especially bovine cells. Therefore, bMyoD was used to directly reprogram the BDFs to bovine myoblasts and myotubes. In the research, the methods for bMyoD delivery were as follows: using a cell-penetrating protein and using a retrovirus. Then, we compared their efficiencies. For effective gene regulation during cell developments, the stability of transcription factors is one of the most significant properties. However, most transcription factors have been demonstrated to have poor stability. Previously, we reported that 30 Kc19-cargo-NLS-R9 protein can be used to deliver cargo proteins into cytoplasm and even into nuclei [49; 50; 51; 52]. In addition, the 30 Kc19 protein was reported to enhance the stability of transcription factors.