Immunoblotting was performed as described19 (link) with the following antibodies: 53BP1 (175933, Abcam; 1:1000; 100-304, Novus Biological; 1:1000); BRCA1 (MAB22101, R+D systems; 1:500); BRCA2 (OP95, Millipore; 1:500); CHK1 (8408, Santa Cruz; 1:1000); pCHK1 (2341, Cell Signaling Technology; 1:1000); CHK2 (611570, BD; 1:1000); Flag-tag (M2, Sigma; 1:1000); γ-Tubulin (GTU488, Sigma; 1:20,000); GFP (11814460001, Sigma; 1:1000); HA (3724, Cell Signaling Technology; 1:20,000); HSP70 (610608, BD; 1:1000); MAD2L2/REV7 (180579, Abcam; 612266, BD; 1:1000); Myc-tag (9B11, Cell Signaling Technology; 1:1000); OBFC1/STN1 (89250, Abcam; 1:1000; SC-376450, Santa Cruz; 1:1000); PRIM1 (10773-1-AP, Proteintech; 1:1000); RIF1 (#1240, de Lange Lab; 1:1000); SHLD1 (PA5-59280, Thermo-Fisher; 1:1000); SNAP tag (9310, NEB; 1:1000); TRF2 (#1254, de Lange Lab; 1:5,000). Affinity-purified peptide antibodies against mouse SHLD1 and SHLD2 proteins (Chapman Lab, unpublished; 1:1000) were generated by Eurogentec.