Described herein is a method of determining lithium responsiveness in a bipolar disorder patient. The method includes obtaining a sample from a patient having bipolar disorder, and assaying the sample for the presence or absence of one or more glutamate decarboxylase-like 1 (GADL1) gene variants selected from the group consisting of a T allele of the single nucleotide polymorphism (SNP) rs17026688, a G allele of the SNP rs17026651, and GADL1 1VS8+48delG. The presence of one or more of the GADL1 gene variants indicates that the patient is responsive to lithium treatment.