To date, the Gabra1 knockout mouse is the only available genetic animal model of ET, although more than 500 mutations are known to cause tremulous behavior in mice and rats (Mammalian Phenotype Browser, http://www.informatics.jax.org/searches/MP_form.shtml; Rat Genome Database, http://rgd.mcw.edu/). This evidence concerns the gene GABRA1 and essential thrombocythemia.