Abstract: Inventors have shown that the protein isthmin-1 (ISM-1) is expressed at kidney level in animal and human model. They have also shown that this protein is expressed on the surface but also intracellular of the circulating leukocytes. They have observed that its expression is increased when a subject suffers from INS, MCN or FSGS compared to healthy controls. Among various causes of nephrotic syndrome, ISM-1 leucocyte expression is dramatically increased in patients with INS, MCN or FSGS. Accordingly, the present invention relates to a method for diagnosing INS, MCN or FSGS in a subject comprising the steps of: i) measuring the membrane expression level of isthmin-1 on the circulating leukocytes in a biological sample obtained from said subject; ii) comparing the expression level measured at step i) with its predetermined reference value, and iii) concluding that the subject suffers from INS, MCN or FSGS when the membrane expression level of isthmin-1 is higher than its predetermined reference value.