PDE4D and schizophrenia: This approach has been used in a large Finnish family cohort, in which DISC1 (disrupted in schizophrenia 1) was previously associated with schizophrenia [1,2], and which led to observation of association with four other genes (NDE1, NDEL1, PDE4B and PDE4D) [3,4] that encode protein binding partners of the DISC1 protein [5–8].