SRC and squamous cell carcinoma: In this study we tested three pyrazolo[3,4-d]pyrimidine derivatives that have already demonstrated potent inhibition of Src activation and cell proliferation: SI 35 revealed anti-proliferative activity toward human epidermoid carcinoma A431 cells [15] and human prostatic cell line PC3 [17]; SI 34 and SI 35 inhibited the proliferation of human medullary and follicular thyroid cancer cells [18,19]; SI 83 reduced the growth rate in human osteosarcoma cells [16].