RPS6KB1 and Global developmental delay: The p70 ribosomal S6 kinase 1 (S6K1) is a downstream target of signalling pathways with known roles in cell growth.1 Deletion of S6K in Drosophila causes a developmental delay and reduced body size due to a general reduction in cell size.2 In mammals, two S6K family members arise from distinct genes, S6K1 and S6K2.