Fam151b is a mammalian homologue of the C. elegans menorin gene, which is involved in neuronal branching. the international Mouse phenotyping consortium (iMpc) aims to knock out every gene in the mouse and comprehensively phenotype the mutant animals. This project identified Fam151b homozygous knock-out mice as having retinal degeneration. We show they have no photoreceptor function from eye opening, as demonstrated by a lack of electroretinograph (eRG) response. Histological analysis shows that during development of the eye the correct number of cells are produced and that the layers of the retina differentiate normally. However, after eye opening at P14, Fam151b mutant eyes exhibit signs of retinal stress and rapidly lose photoreceptor cells. We have mutated the second mammalian menorin homologue, Fam151a, and homozygous mutant mice have no discernible phenotype. Sequence analysis indicates that the FAM151 proteins are members of the pLc-like phosphodiesterase superfamily. However, the substrates and function of the proteins remains unknown.The retina is the light sensitive tissue located in the posterior eye. Photoreceptor cells within the retina are responsible for converting light signals into electrical signals, and loss of these cells is a leading cause of blindness in humans. In the course of diseases such as retinitis pigmentosa (RP), rod and cone photoreceptor cells are slowly lost, beginning at the peripheral retina and moving centrally 1 . Multiple mutant genes in humans can cause RP, and many of these have counterpart mouse models 2 . In this paper we describe a novel mouse model of retinal degeneration which has early and rapid loss of photoreceptor cells due to loss of function of a largely uncharacterised gene. The International Mouse Phenotyping Consortium (IMPC) is a worldwide collaborative effort to generate lines of mice, each with a loss of function mutation in a single gene, and to comprehensively phenotype the lines 3,4 . The ultimate aim is to catalogue the function of every gene in the mouse genome in order to contribute to the understanding of fundamental mammalian biology and to generate models for human disease. Many institutions participate and follow an agreed pipeline for the phenotypic analysis of the mice, known as IMPReSS (International Mouse Phenotyping Resource of Standardised Screen). The analyses include growth, morphological, behavioural, metabolic and sensory phenotypes over a 15 week period, and followed post-mortem by examination of haematology, immunology, clinical chemistry and gross pathology. At 15 weeks of age mutant mice undergo slit lamp examination and ophthalmoscopy to investigate the gross morphology of the eye.As part of this study a novel gene, Fam151b, was mutated and the mice analysed through the IMPC pipeline. Fam151b knockout mice were found to have abnormal retina morphology and no other observed phenotype.Here we follow up these findings with a detailed histological and functional analysis of this disease model. In addition we have mutated the paralogous...