PURPOSE. The Rs1h knockout mouse displays retinal features typical for X-linked juvenile retinoschisis (RS). Consequently, this mouse line represents an excellent model to study early molecular events in RS. METHODS. Whole genome expression profiling using DNA-microarrays was performed on total RNA extracts from retinoschisin-deficient and wild-type murine retinas from postnatal days 7, 9, 11, and 14. Quantitative real-time RT-PCR (qRT-PCR) analysis of additional time points facilitated the refinement of the temporal expression profile of differentially regulated transcripts. Differential protein expression was confirmed by Western blot analysis. RESULTS. Based on biostatistic and knowledge-based DNA- microarray analyses we have identified differentially regulated retinal genes in early postnatal stages of the Rs1h-deficient mouse defining key molecular pathways including adhesion, cytoskeleton, vesicular trafficking, and immune response. A significant upregulation of Egr1 at P11 and several microglia/glia-related transcripts starting at P11 with a peak at P14 were identified in the diseased retina. The results provided evidence that macrophage/microglia activation precedes apoptotic photoreceptor cell death. Finally, the role of Egr1 in the pathogenesis of Rs1h-deficiency was investigated, and the results indicated that activation of the MAPK Erk1/2 pathway occurs as early as P7. Analyses of Rs1h(-/Y)/Egr1(-/-) double-knockout mice suggest that Egr1 upregulation is not a prerequisite for macrophage/microglia activation or apoptosis. CONCLUSIONS. The findings imply that microglia/glia activation may be triggering events in the photoreceptor degeneration of retinoschisin-deficient mice. Furthermore, the data point to a role of Erk1/2-Egr1 pathway activation in RS pathogenesis.