An in vitro method for diagnosing depression disorders, 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 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 PRKCB, MDM2, PIAS1, IFNAR1, IFNAR2, LYN, AHR, RASSF1, GAB2, CAMK1D, KCNJ15, IL17RA, PTPRC 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 the final result value or depression score obtained for the patient and a control result value obtained for a healthy control subject, wherein the control result value was determined in a manner comparable to that of the final result value; and d) if said final result or depression score value is greater than a threshold/cut-off, classifying said patient as being positive or having depression disorders, or, if said final result value is not greater than said threshold, classifying said patient as being negative or not having depression disorders. a) determining for a combination of at least two A to I editing RNA 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 PRKCB, MDM2, PIAS1, IFNAR1, IFNAR2, LYN, AHR, RASSF1, GAB2, CAMK1D, KCNJ15, IL17RA, PTPRC 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; 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 the final result value or depression score obtained for the patient and a control result value obtained for a healthy control subject, wherein the control result value was determined in a manner comparable to that of the final result value; and d) if said final result or depression score value is greater than a threshold/cut-off, classifying said patient as being positive or having depression disorders, or, if said final result value is not greater than said threshold, classifying said patient as being negative or not having depression disorders.