Mesenchymal stem (stromal) cells (MSC) have potent immunomodulatory properties potentially promoting allograft tolerance and ameliorating toxicity of exposure to high dose CNI. Previous trials for non-solid organ transplant indications have shown an excellent safety profile of intravenous MSC application. The MYSTEP1 trial aims to investigate safety and benefits portal and intravenous MSC infusion in pediatric LT.