An in vitro method for differential diagnosing bipolar disorder and unipolar depression in a human patient from a biological sample of said patient, said method comprising: a) determining for a combination of at least two A to I editing RNA: - for each selected biomarker, the relative proportion of RNA editing at a given editing site for at least one or a combination of sites which can be edited on the RNA transcript of said at least two biomarkers, and/or - the relative percentage of an isoform or of a combination of isoforms of the RNA transcript of each of said two biomarkers; wherein said at least two A to I editing RNA biomarkers are selected from the group consisting of MDM2, PRKCB, PIAS1, IFNAR1, LYN, AHR, RASSF1, GAB2, CAMK1D, IL17RA, PTPRC, KCNJ15, IFNAR2 and PDE8A biomarkers; b) determining a result value obtained for each of said at least two biomarkers and a final result value based on an algorithm or equation that includes the result value obtained for each selected biomarker; c) determining whether said result value obtained in step b) is greater or not greater than a control value obtained for control unipolar subject, wherein the control value was determined in a manner comparable to that of the result value; and d) if said result value obtained for the patient is greater than a threshold, classifying said patient as having bipolar disorder or, if said result value is not greater than said threshold, classifying said patient having unipolar depression. a) determining for a combination of at least two A to I editing RNA: - for each selected biomarker, the relative proportion of RNA editing at a given editing site for at least one or a combination of sites which can be edited on the RNA transcript of said at least two biomarkers, and/or - the relative percentage of an isoform or of a combination of isoforms of the RNA transcript of each of said two biomarkers; wherein said at least two A to I editing RNA biomarkers are selected from the group consisting of MDM2, PRKCB, PIAS1, IFNAR1, LYN, AHR, RASSF1, GAB2, CAMK1D, IL17RA, PTPRC, KCNJ15, IFNAR2 and PDE8A biomarkers; - for each selected biomarker, the relative proportion of RNA editing at a given editing site for at least one or a combination of sites which can be edited on the RNA transcript of said at least two biomarkers, and/or - the relative percentage of an isoform or of a combination of isoforms of the RNA transcript of each of said two biomarkers; wherein said at least two A to I editing RNA biomarkers are selected from the group consisting of MDM2, PRKCB, PIAS1, IFNAR1, LYN, AHR, RASSF1, GAB2, CAMK1D, IL17RA, PTPRC, KCNJ15, IFNAR2 and PDE8A biomarkers; b) determining a result value obtained for each of said at least two biomarkers and a final result value based on an algorithm or equation that includes the result value obtained for each selected biomarker; c) determining whether said result value obtained in step b) is greater or not greater than a control value obtained for control unipolar subject, wherein the control value was determined in a manner comparable to that of the result value; and d) if said result value obtained for the patient is greater than a threshold, classifying said patient as having bipolar disorder or, if said result value is not greater than said threshold, classifying said patient having unipolar depression.