Also, in the case of a doxorubicin-resistant canine hemangiosarcoma cell line established to study drug resistance, it was found that the DDR pathway was attenuated, as the mRNAs for ATM, ATR, and Chk1 were significantly decreased, suggesting a possible role for ATR in doxorubicin resistance (15). This evidence concerns the gene CHEK1 and angiosarcoma.