PER2 and breast cancer: One study of tumour tissues from breast cancer patients found hypermethylation on the promoters of PER1, PER2, CRY1 or BMAL1 genes in about 70% of the cases, compared to <50% methylation in non-cancerous tissues.[54] In the same study, homogeneous (non-rhythmic) expression of PER2 or BMAL1 was significantly associated with lymph node metastasis and poorer prognosis for the patient.