Analyses of first exon expression. (A) Schematic representation of the splicing patterns from exon 1 to its downstream 3′ ss. p3′ss indicates proximal 3′ss; d3′ss, distal 3′ss. AUG-1 located within exon 2′. (B) First exon expression in human and mouse cell lines and tissues. A common anti-sense primer annealing to exon 2 and variable forward primers specific to each of the first exons were used for PCR on cDNA synthesized by use of a downstream exon 2 primer. The top panels are PCR; the bottom panels are Southern hybridization with exon 2′ as a probe (Ex2′). (C) First exon expression during cultured CD34⁺ erythroid differentiation by use of the same RT-PCR strategies. The top panels are PCR products; the bottom panels are Southern-blot hybridizations that use exon 2′ as a probe (Ex2′). (D) Real-time PCR analysis of ratios of 1A:1C expression during CD34⁺ differentiation. 0, 4, 8, 10, and 14 indicate days of differentiation.