Methylation of β-globin promoter sequences. (A) Diagram of the human β-globin promoter region showing the locations of binding sites for transcription factors and CpG dinucleotides. Adapted from Stamatoyannopoulos and Grosveld.²⁶  (B) Representative methylation data for the β-globin promoter in fetal and adult erythroid cells. □ indicates unmethylated; ■, methylated; and ▩, indeterminate sequencing. (C) Summary methylation data from multiple independent tissue samples. Error bars indicate plus or minus 1 standard deviation.