In the case of the Aldh1a family, although RA is important in human disease, we still know little about the spatio-temporal dynamics of the expression domains and functions of ALDH1A1, ALDH1A2 and ALDH1A3 genes during human development and adult organ homeostasis, other than RT-PCR studies [98], which do not provide enough resolution at the single cell level to understand how the sources of RA regulate physiological action. This evidence concerns the gene ALDH1A3 and rheumatoid arthritis.