PITX1 and premenstrual tension: We did observe an imbalance in the Ka/Ks ratio between PMS cases and controls, with PMS cases exhibiting a greater than expected ratio of non-synonomous:synonymous variation, which may indicate that ccf-mtDNA export may be a selective process; where the ‘wild-type’ mtDNA molecule is retained in the cell and the ‘mutant’ mtDNA molecule is preferentially exported.