IGF2 mRNA expression relative to β-ACTIN mRNA expression in the mammary gland. IGF2 mRNA expression (mean ± standard error of the mean) compared with β-ACTIN expression in the mammary gland across the four phases of lactation. IGF2 was highest in the perinatal period, but steadily decreased throughout the remainder of lactation. a significantly higher than b(P < 0.05). IGF, insulin-like growth factor.