This chapter is a Luhmannian reading of some early ethical writings about 'saviour siblings"; that is, where children with serious medical disorders are treated by tissue transplanted from a sibling, where the embryo-that-becomes-the-sibling has been genetically screened and selected before implantation to ensure that it is a donor-compatible tissue match. It is the first, and so far the only, instance of the 'screening in" of desirable genetic characteristics allowed within the UK regulatory framework.