KLK6 and atrial fibrillation: Lactate dehydrogenase (LDH) release, an indicator of cell lysis, qPCR, and WST-1 assay showed that cells with high KLK6 mRNA and protein expression levels showed dose- and time-dependent resistance to AF when compared to cells with lower KLK6 expression (Figure 2A and Supplementary Figure S2A-S2C).