HELLS and ICF syndrome: In addition to the rough features, a recent genome‐wide DNA methylation analysis using the Illumina Infinium HumanMethylation450 BeadChip (HM450K) revealed that the detailed genome‐wide DNA methylation patterns are also different between patients with ICF1 and those with other types of ICF syndrome; CpG islands are preferentially hypomethylated in the former, whereas CpG poor regions with heterochromatic and late‐replicating signatures are hypomethylated in patients with ICF2, ICF3, and ICF4 (Table 2) (Velasco et al., 2018).