The main findings of the present study were that 1) SMAD3 and STAT3 may have the highest diagnostic potential in RA patients classification, 2) positive correlation between SMAD3 and STAT3 in RA patients was observed, 3) STAT5a expression was not detected in Th17 cells, 4) Treg/Th17 ratio index in RA patients’ blood was lower than in HC blood, clearly indicating a disturbance in Th response, 5) the range of IL-17 and IL-21 serum levels were higher in RA patients than in OA patients as well as higher serum IL-2 and IFN-γ levels in RA and OA patients than in healthy subjects. The gene discussed is SMAD3; the disease is rheumatoid arthritis.