Standard curves for the determination of efficiency (E) and correlation coefficients (R2) of six candidate reference genes of Atractosteus tropicus for qPCR. (A) alfa elongation factor (ef1- α), (B) beta-actin (actb), (C) 18S ribosomal RNA (18s rrna), (D) beta-2-microglobulin (b2m), (E) Tubulin alpha (α- tub) and (F) Glyceraldehyde 3-phosphate dehydrogenase (gapdh).