**3.3 Genetic labeling of somatic cells for the investigations of therapeutic mechanisms**

Genetically modified animal models with fluorescent marker genes by SCNT are highly useful as they enable a more efficient monitoring method of cell survival and cellular kinetics *in vivo* after cell product transplantation [53, 56]. Matsunari et al. produced transgenic-cloned pigs carrying the humanized Kusabira-Orange (huKO) gene, yielding an orange-red fluorescence in its dimeric form. The clear red fluorescence of the huKO protein is maintained in paraffin-embedded tissue sections. The pigs express fluorescent protein not only in various organs but also in pig stem cells or progenitor cells [57].

In hepatocytes or liver organoid transplantation experiments, donor cells/organoids were derived from transgenic KO-expressing pigs and transplanted KO-negative littermate, which showed the distribution and survival of transplanted materials [58, 59]. These techniques are also used in allograft ligament construction to analyze intrinsic and extrinsic cellular dynamics during graft healing [60]. It would also be applicable in research of cardiac regenerative medicine using stem cells in which the tracing of survived cells after transplantation and the mechanisms of graft and host interaction have been investigated by 3-dimensional (3D) imaging of heart tissue after tissue clearing using light-sheet microscopy [61, 62]. Upon distinguishing recipient and donor cells in transplanted grafts, KO-expressing pigs may become sufficient genetic modification models for the 3D posttransplantation analysis of vascular network formation inside of the graft.
