In line with the finding that plasma CTSD activity was significantly higher in male type 2 diabetes compared with healthy males, we found that plasma CTSD activity positively correlated with HbA1c (%) (Figure 3B; r = 0.577, p = 0.002), HOMA-IR (Figure 3D; r = 0.398, p = 0.027) and plasma glucose (Figure 3F; r = 0.465, p = 0.015). Here, CTSD is linked to type 2 diabetes mellitus.