Given the known status of gut dysbiosis in PKU following Phe-restricted diet [43], the early modulation of the gut microbiota, also achievable through the addition of biotics (e.g., HMOs, GOS/FOS respecting 9:1 ratio) to infant protein substitutes, could be considered to have beneficial effects on dysbiosis and microbiota–gut–liver–brain axis, although results from clinical intervention studies are needed. This evidence concerns the gene FOS and phenylketonuria.