Human histidyl-tRNA synthetase (HARS) is an aminoacyl-tRNA synthetase (AARS) that catalyzes the attachment of the amino acid histidine to histidyl-tRNA (tRNAHis) in a two-step reaction that is essential for protein translation. Currently, two human diseases, Usher Syndrome IIIB (USH3B) and an inherited peripheral neuropathy, Charcot Marie Tooth Syndrome (CMT), have been linked genetically to single point mutations in the HARS gene. The recessive HARS USH3B mutation encodes an Y454S substitution localized at the interface between the anticodon-binding domain and the catalytic domain of the opposing subunit. Patients with Usher Syndrome IIIB lose their sight and hearing during their second decade of life, and clinicians have observed that the onset of deafness and blindness may be episodic and correlate with febrile illness. Furthermore, some young USH3B patients present with a fatal form of acute respiratory distress. In addition to the single HARS mutation linked to Usher Syndrome, eight other mutations in the HARS gene are associated with CMT, an inherited peripheral neuropathy. Peripheral neuropathies are associated with progressive and length-dependent damage of the motor and sensory neurons that transmit information to the spinal cord. The age of onset and phenotypic severity of CMT linked to HARS is highly variable. When expressed in a yeast model system, the HARS variants are dominantly lethal, and confer defects in axonal guidance and locomotor deficiencies when expressed in C.elegans. Here, the biochemical characterization of the HARS USH3B and three peripheral neuropathy variants are described. The approaches included enzyme kinetic analysis with purified HARS enzymes to monitor catalytic deficiencies, differential scanning fluorimetry (DSF) to evaluate structural instability, and cellular models to detect physiological effects of axonal outgrowth by CMT variants. The results suggest that Usher Syndrome IIIB is unlikely to be a consequence of a simple loss of aminoacylation function, while HARS-linked peripheral neuropathy variants all share common catalytic defects in aminoacylation. The HARS system represents a notable example in which two different complex human diseases arise from distinct mutations in the same parent gene. By understanding the biochemical basis of these inherited mutations and their link to Usher Syndrome and CMT, it may be possible to develop mechanism-based therapies to improve the quality of life of patients afflicted with them.