In Euro-American kinship biogenetic substance has been considered as a fact of nature as well as a symbol of diffuse and enduring solidarity. In this paper I analyse how changes in the facts of biogenetic substances induced by research, and the application of genetics in human reproduction, imply also changes in their capacity to symbolise social solidarity of kinship. Is nature still a symbol of kinship? If so, what kind of human nature is a biotech nature? The ethnography is based on fieldwork in two clinics of assisted reproduction in Barcelona (Spain), mainly among egg donors and receivers. I analyse the ideas of inheritance, identity and similarity between members of a family and attempt to answer the following questions. What does it mean to share biogenetic substances in the context of assisted reproduction? How do egg donors understand the genetic link with an unknown offspring? How do they understand the biogenetic substance made with their donated eggs? How do receivers understand the biogenetic substance made in the laboratory? How do they appropriate the biogenetic substance? How do they relate genetic links, gestational links and descent links?