Study of the genetics of the malignant hyperthermia syndrome began in families in which both malignant hyperthermia (MH) episodes had been experienced and individuals had strongly positive contracture tests diagnostic of susceptibility to MH. Linkage studies associated this MH phenotype to the ryanodine receptor gene (RYR1) at chromosome 19q13.1 in many families. Although the MH phenotype is not always linked to chromosome 19, the RYR1 has remained the focus of experimentation. Other candidate genes exist, but few MH-susceptible families have variants of these genes. Hundreds of MH-susceptible people have variants of RYR1.