This is the author accepted manuscript. The final version is available from the Society of Neuroscience via http://dx.doi.org/10.1523/JNEUROSCI.1509-14.2015 ; Recent studies on the pathogenic mechanisms of recessive hyperekplexia indicate disturbances in glycine receptor (GlyR) ?1 biogenesis. Here, we examine the properties of a range of novel glycine receptor mutants identified in human hyperekplexia patients using expression in transfected cell lines and primary neurons. All of the novel mutants localized in the large extracellular domain of the GlyR ?1 have reduced cell surface expression with a high proportion of receptors being retained in the ER, although there is forward trafficking of glycosylated subpopulations into the ER-Golgi intermediate compartment and cis-Golgi compartment. CD spectroscopy revealed that the mutant receptors have proportions of secondary structural elements similar to wild-type receptors. Two mutants in loop B (G160R, T162M) were functional, but none of those in loop D/?2?3 were. One nonfunctional truncated mutant (R316X) could be rescued by coexpression with the lacking C-terminal domain. We conclude that a proportion of GlyR ?1 mutants can be transported to the plasma membrane but do not necessarily form functional ion channels. We suggest that loop D/?2?3 is an important determinant for GlyR trafficking and functionality, whereas alterations to loop B alter agonist potencies, indicating that residues here are critical elements in ligand binding. ; This work was supported by the Deutsche Forschungsgemeinschaft (Grant DFG VI586 to C.V.) and the European Union (FP7 project Neurocypres to C.J.K., K.L.P., and S.C.R.L.). N. Schaefer and G.L. are supported by the GSLS Wuerzburg. S.C.R.L. is a Wellcome Trust Senior Research Fellow in Basic Biomedical Research.