HP1β plays a role in H4K20me3 but not H3K9me3 spreading from ERVs into flanking genomic DNA. N-ChIP was performed with H3K9me3-, H4K20me3- and pan-H3-specific antibodies using chromatin isolated from HM1, Cbx5-/- and Cbx1-/- mESCs. The level of enrichment of these modifications at the flanks of two full-length IAP elements on chromosomes 2 and 5 as well as at positions approximately 1 kb, 2 kb and 3.5 kb distal to these flanking regions, was determined by qPCR. Data are presented as means ± SD of three technical replicates, and pairs of control and experimental samples with *P < 0.05 and **P < 0.01 (two-tailed Student's t-test) are shown. H3K9me3 enrichment levels across these genomic regions as determined using our previously published ChIP-seq data sets  are also shown for wt and Setdb1 KO mESCs.