Hoxa-9 in day-15.5 fetal thymuses. (A) Microphotographs of sections of mutant (top) and normal (bottom) day-15.5 fetal thymuses stained with methylene blue (original magnification × 150). Sections were taken through the midportion of each gland. (B) Hoxa-9 gene expression in normal fetal thymus. RT-PCR using primers specific for Hoxa-9 was performed on day-15.5 fetal tissues and the amplified cDNAs probed for Hoxa-9by Southern blot analysis. Lanes 1 through 5 with RT; lane 1, thymus; lane 2, liver; lane 3, brain; lane 4, heart; lane 5, whole embryo; lane 6, markers (M); lanes 7 through 11, same tissues with no RT; lane 12, control (C) with no DNA. Strong signal is seen in lanes 1 and 2, representing fetal thymus and liver, respectively. (Upper panel) Ethidium bromide staining. (Middle panel) Southern blotting forHoxa-9. (Lower panel) Actin control.