PDE6H and retinal degeneration: Group (b) was very small with only 9 genes, and it did not make a cluster of functionally related GO terms, although 6 of 9 genes were annotated as causal genes for retinal degeneration (Abca4, Elovl2, Fscn2, Nxnl1, Pde6c, and Pde6h).