B2M and premenstrual tension: To investigate further, we tested the effect of B2M level on ccf-mtDNA level in PMS using logistic regression, confirming our previous association between PMS and ccf-mtDNA when including B2M level as a covariate (p = .004), identifying no significant correlation between B2M levels and ccf-mtDNA levels (r2 = 2.5 × 10−7 and p = 99, Supplementary Fig. 1) and no significant difference in B2M levels between PMS cases and controls (Wilcoxon Ranked Sum Test p > .05).