The rs13143308T risk variant increases the calcium spark density. (A) Longitudinal plane of a human atrial myocyte from a patient without a 4q25 risk variant, with indication of two spark sites on the left (colored rectangles). Calcium signals are shown for each spark site on the right. Insets show calcium signals for individual sparks. (B) Sparks in a human atrial myocyte from a patient with a 4q25 risk variant. (C) Spark frequency. (D) Spark sites per cell. (E) Spark firing frequency. P-values from the unpaired t-tests (E) or Mann–Whitney tests (C and D) are given above data points and the number of patients in parentheses.