A stimulator protein, homologue or derivative thereof for use in the stimulation of oocyte development is provided. The stimulator protein is selected from the group consisting of midkine, pleiotrophin, CO6_HUMAN Complement component C6, SSPO_HUMAN SCO-spondin, SPON1_HUMAN Spondin-1, UNC5B_HUMAN Netrin receptor UNC5B, BAI1_HUMAN Brain-specific angiogenesis inhibitor 1, UNC5C_HUMAN Netrin receptor UNC5C, SEM5B_HUMAN Semaphorin-5B and PROP_HUMAN Properdin. Related methods are also disclosed.
