Death of spinal motor neurons is a hallmark of devastating and currently untreatable neurodegenerative diseases. However, little is currently known about the reasons why motor neurons are particularly sensitive to mutations affecting broadly expressed genes, such as SOD1 in amyotrophic lateral sclerosis or SMN in spinal muscular atrophy. It is assumed that cell intrinsic differences in motor neurons underlie the disease susceptibility, but nature of such modifiers on neuronal survival is currently poorly understood. Here we propose to study a recently generated mutation of a mir-17~92 cluster of micro RNAs. The deletion of this cluster results in a striking loss of limb muscle innervating spinal motor neurons, while other spinal neuronal classes appear unaffected. We propose to: first, study the cellular pathology of mir-17~92 null animals to determine whether any other cells besides motor neurons are dying in mir-17~92 null spinal cords and whether individual motor neuron subtypes exhibit different degree of degeneration. We will examine whether motor neuron cell death is due to cell autonomous function of miRNAs or due to the loss of miRNAs in other cells that might impinge on motor neuron survival in a non-cell autonomous fashion. Finally, we will determine whether miRNAs are required both in progenitors and postmitotic neurons to exert their pro-survival function. Second, we propose to dissect which specific miRNAs from the mir-17~92 cluster are involved in motor neuron death and which genes are deregulated in the absence of the miRNAs. We will test whether previously identified pro-apoptotic targets are involved in motor neuron degeneration. Furthermore, we will perform unbiased expression screen and test the function of selected identified biochemical pathways in motor neuron survival. As a lead into future studies we propose to determine whether overexpression of the miRNA cluster might save motor neurons from naturally occurring programmed cell death. We expect that these studies will define cellular pathologies in the developing spinal cord connected to the loss of mir-17 ~ 92 clusters and identify relevant molecular pathways linking the miRNAs and motor neuron survival. Understanding miRNA controlled cell type specific survival pathways might provide new targets for slowing down or arresting progression of motor neuron loss in motor neuron diseases.