A slight effect (ca. 15%) was seen at a concentration of 150 µg/ml for RADA-KGHK for both cell lines and for RADA-GHK in 46BR.1N fibroblasts. The results obtained in the XTT analysis of the effects of the peptides on proliferation of human 46BR.1N fibroblasts and HaCaT keratinocytes (Fig. ) showed that the peptides stimulated the proliferation of human skin cells. The most prominent pro-proliferating effect can be observed at concentrations of 1-50 ug/ml. This effect was slightly stronger for the HaCaT keratinocytes cell line, whose proliferation was increased by 30-45% compared to the control at concentrations of 10-50 µg/ml. In both cases, the pro-proliferating effect was more potent after 72 h of incubation (48h incubation graph can be found in Suplementary Data Fig. ) , which was consistent with long-term application. In addition, the peptides did not cause inhibition of proliferation of both tested cell lines.