Knockout of ATM was achieved by CRISPR-Cas9 genome editing using LentiArray™ Lentiviral sgRNA (Product # A32042, Assay ID CRISPR901163_LV) and LentiArray Cas9 Lentivirus (Product # A32064). Western blot analysis of ATM was performed by loading 30 µg of HeLa Wild Type (Lane 1), HeLa Cas9 (Lane 2) andHeLa ATM KO (Lane 3) whole cell extracts. The samples were electrophoresed using NuPAGE™ 3-8% Tris-Acetate Protein Gel (Product # EA0378BOX). Resolved proteins were then transferred onto a nitrocellulose membrane (Product # IB23001) by iBlot® 2 Dry Blotting System (Product # IB21001). The blot was probed with Anti-ATM Polyclonal Antibody, DyLight 488 (Product # PA5-22690, 1:1,000 dilution) and Goat anti-Rabbit IgG (H+L) Superclonal™ Recombinant Secondary Antibody, HRP (Product # A27036, 1:4,000 dilution) using the iBright FL 1000 (Product # A32752). Chemiluminescent detection was performed using Novex® ECL Chemiluminescent Substrate Reagent Kit (Product # WP20005). Loss of signal upon CRISPR mediated knockout (KO) using the LentiArray™ CRISPR product line confirms that antibody is specific to ATM.