Described herein are processes and devices useful for the identification of genes differentially expressed in skin. Preferably, the processes and devices are useful for the in vitro determination of skin homeostasis. Also described is a combination having a plurality of genes differentially expressed in skin compared to other tissues. Biochips and test kits useful in processes for the identification of genes expressed in skin homeostasis as well as in processes for screening substances useful in maintaining or promoting homeostasis of the skin are also described. Also described are processes for identifying substances useful in maintaining or promoting homeostasis of the skin as well as processes for making cosmetic and/or pharmaceutical preparations comprising such substances.