CTSL and cancer: Since previous reports showed that CTSL can proteolytically cleave CUX1 to produce a truncated, but more active form of CUX1 with an apparent molecular weight of 110 kDa, instead of 200 kDa for the full length CUX1, in human cancer cells [10, 14, 15], we therefore hypothesized that CTSL expression should be lower in p10 human ECs than that in p5 human ECs.