Six autosomal recessive mutations affecting the CNS, PNS or skeletal muscle of laboratory mouse have been or are being transferred by repetitive backcrosses to each of two inbred strains: C57BL/6J and 129/J. Insterstrain hybrid homozygotes possess increased vigor and survival duration. Mutant organisms on either inbred strain or hybrid background are made available to several laboratories as well as to our own for analysis of phenotypic expression including the cellular locus and sequences; earliest intracellular locus of gene action; philotropic effects and the metabolic deficiencies responsible. The mutations under study include: wl - Wabbler-Lethal. With optic nerve atrophy, primary and secondary vestibular and brain stem reticular formation degeneration; dt - Dystonia Musculorum. With medullary and pontine reticular formation degeneration, DRG cell, especially stretch afferent degeneration. In late stages some lower motor neuron degeneration occurs; dtH - Dysafferent. This allele of dt is similar but with milder expression; wr - Wobbler. With lower motor neuron perikaryon and axonal degeneration. cg - Ching Luan. With peripheral vestibular and cerebellar dysfunction. dy - Dystrophia Muscularis. With primary myopathy of skeletal muscle, most severe in distal extremities. Histochemical, neuroanatomical, TEM and SEM and electrophysiological methods are employed in studies of these mutations, with varying emphasis. Current analyses will be continued with increased emphasis on enzyme histochemical and EM methods.