MFAP4 and triple-A syndrome: To identify Mfap4-expressing cells, we performed in situ hybridization staining for Mfap4 transcript that revealed that Mfap4 mRNA is expressed in adventitial cells (presumably adventitial fibroblasts) and medial SMCs and that it is upregulated upon AAA infusion (Figure 1D and Supplementary Figure 3).