This pathogenicity assessment is relevant only for malignant hyperthermia susceptibility (MHS) inherited in an autosomal dominant pattern. Variants in RYR1 can also cause other myopathies inherited in an autosomal dominant pattern or in an autosomal recessive pattern. Some of these disorders may predispose individuals to malignant hyperthermia. RYR1 variants may also contribute to a malignant hyperthermia reaction in combination with other genetic and non-genetic factors and the clinician needs to consider such factors in making management decisions. This sequence variant predicts a substitution of asparagine with serine at codon 1678 of the RYR1 protein p.(Asn1678Ser). The maximum allele frequency for this variant among the six major gnomAD populations is SAS:0.000033, a frequency consistent with pathogenicity for MHS. This variant has been reported in one individual who had a personal or family history of a malignant hyperthermia reaction and a positive in vitro contracture test (IVCT) result (when the proband was unavailable for testing a positive diagnostic test result in a mutation positive relative was counted) (PMID:30236257), this individual was also reported to have a pathogenic variant in RYR1, p.Gly2434Arg, and was not considered for PS4 (personal communication, The UK (Leeds) MH Unit). No functional studies were identified for this variant. This variant does not reside in a hotspot for pathogenic variants that contribute to MHS (PMID: 21118704). A REVEL score <0.5 supports a benign status, BP4. Based on using Bayes to combine criteria this variant is assessed to be a Variant of Uncertain Significance, (PMID: 29300386). Criteria implemented: BP4.