MASL1 expression is upregulated during primary human CD34⁺ cell erythroid differentiation and is an abundant protein in human red blood cells (RBCs). Human peripheral blood CD34⁺ cells were expanded for 6 days and were then induced to differentiate by EPO treatment during a 14-day period. (A) MASL1 gene expression was examined in CD34⁺ cells at day 0, 3, 5, 7, 10, and 14 after EPO treatment by semiquantitative RT-PCR. GAPDH was used as an internal control. (B) Mean relative MASL1 expression levels shown as fold induction compared with levels in CD34⁺ cells at day 0 by qRT-PCR. Values were normalized to the expression level of the housekeeping gene GAPDH. Error bars represent the SD from 3 individual experiments; *P < .05; **P < .01. (C) Western-blot analysis of protein lysates of EPO-induced CD34⁺ cells using anti-MASL1, hemoglobin-α, and GPA antibodies. β-actin was used as an internal control. (D) Western-blot analysis of protein lysates prepared from CD34⁺ cells at day 14 of EPO-induced differentiation (EPO Day 14) and human RBCs. GAPDH was used as an internal control. (E) Mean relative MASL1 protein expression level normalized to GAPDH. Error bars represent the SD from 3 individual experiments; *P < .05.