PCs showed distinct CpG density category enrichments. Enrichment and depletion of CpG density categories in subsets of CpG sites with < − 1σ projections (left bar) and > 1σ projections (right bar) compared to the background total 450K CpG sites (central bar). (A-C) Tissue-related PC1 to PC3 showed an enrichment of LC probes and depletion of HC probes irrespective of the sign of the projection. (D) PC4, the age pattern present in all tissues checked except blood, showed an enrichment of HC probes and depletion of LC probes in both positive and negative projections. (E) PC5, the age pattern present in all the tissues checked, was enriched in HC probes for positive projections (CpGs show increased methylation with age) and was enriched in LC probes for negative projections (CpGs show decreased methylation with age). HC, high-density CpG island; IC, intermediate density CpG island; LC, low-density CpG island; PC, principal component.