Kiss I, Pospisilova E, Kolostova K, et al. - Researchers focused on the occurrence and molecular features of circulating endometrial cells (CECs) in 20 women with spontaneous pneumothorax (SP) with high doubt of its catamenial character. Via size-based separation (MetaCell), CECs were enriched from peripheral blood. They performed gene expression profiling of captured cells, in addition to cytomorphology, for 24 endometriosis-related genes. The presence of CECs was reported in all participants. Two different phenotypes of CECs in SP and/or catamenial pneumothorax were identified via gene expression profiling: one referring to the diaphragm openings syndrome and the other to endometrial tissue pleural implantations. On comparing gene expression profiles of CECs in pneumothorax (CECs-SP group) vs CECs in pelvic endometriosis (CECs-non-SP group), they found that the CECs-SP group demonstrated a significantly higher expression of HER2 than the CECs-non-SP group. In patients with SP, the isolation, as well as characterization of CECs, could be successfully accomplished in this proof-of-concept study. Endometriosis involvement could be signaled by the identification of CECs in SP, which also aids early referral to gynecologic consultation for more examination and treatment.