Transcripts encoding TH and DAT are downregulated in the ileum of mice that lack TPH2 or TPH1 and TPH2. Real-time PCR was used to analyze TH and DAT expression quantitatively in the ileum. A, Expression of TH in TPH1KO, TPH2KO, and TPH1/2dKO mice was compared, respectively, with that in their wild-type littermates; data are expressed as percentage WT. Transcripts encoding TH were significantly less abundant in TPH2KO and TPH1/2dKO animals than in their wild-type littermates. The abundance of transcripts encoding TH in TPH1KO mice, however, did not differ significantly from that in their wild-type littermates. TH expression in TPH1/2dKO animals was not significantly different from that in mice lacking only TPH2. B, Expression of DAT in TPH1KO, TPH2KO, and TPH1/2dKO mice was compared, respectively, with that in their wild-type littermates; data are expressed as percentage WT. The abundance of transcripts encoding DAT in TPH2KO and TPH1/2dKO animals was significantly less than that in their wild-type littermates. The abundance of transcripts encoding DAT in TPH1KO mice, however, was greater than that in their wild-type littermates. DAT expression in TPH1/2dKO animals was not significantly different from that in mice lacking only TPH2. ns, Not significant.