MKI67 and atrial fibrillation: Pituitary tumor cells from AF males exhibited stem‐like cell properties and showed elevated expression of stem cell regulatory genes and proteins (SOX‐2, OCT‐4, KLF4, SNAIL‐1, and Nestin), tumor aggressiveness markers (MMP‐9, CD44, CD34, PTTG, FGFR4, Ki‐67, N‐Cadherin), and prolactin compared to those from AD and PF controls.