Promoter methylation could be involved in tissue specific silencing of these genes and had been reported for the Hoxa5 gene in lung cancer [126], in human breast tumors [127], in myeloid and lymphoid malignancy [128] and for the Hoxb5 gene in ovarian carcinomas [129] where the gene undergoes de novo methylation. This evidence concerns the gene HOXB5 and lung cancer.