Primer extension of ALAS2 poly (A)⁺ RNA. Electrophoretic separation of primer extension products was carried out in denaturing polyacrylamide gel electrophoresis as described in “Patients and methods.” Lanes 1 and 6 are primer extension products of poly (A)⁺ RNA by an ALAS2 exon 1 primer. Lane 7 is an extension control with no template. Lanes 2 to 5 and 8 to 11 are sequencing extension products from a genomic ALAS2 clone.