A method of classifying a subject having idiopathic scoliosis (IS) or at risk of developing IS comprising: determining the cellular response to Gi stimulation in a cell sample from the subject in the presence of OPN; determining the cellular response to Gi stimulation in a cell sample from the subject in the absence of OPN; and comparing the cellular response obtained in the presence of OPN with the cellular response obtained in the absence of OPN, whereby the comparing step enables the classification of the subject into one IS functional group. Also provided is the use of the foregoing method to classify borderline subjects and kits for applying the methods.