Delivery of mRNA in vivo. (a) LNPs functionalized with antibodies for CD5, a marker for T cells, loaded with mRNA encoding a CAR designed against the FAP. Once the mRNA is released from the endosome, it is translated into a functional CAR and the newly engineered CAR-T cells target and kill activated fibroblasts. (b) LNPs functionalized with antibodies against c-KIT, a marker for mouse HSCs, were filled with mRNA encoding the enzyme Cre recombinase. HSCs in the transgenic mouse model harboring a loxP-flanked STOP cassette that prevents the expression of tdTomato were targeted. In the absence of Cre recombinase, the cells do not express tdTomato and are not red. However, when Cre is delivered to the cells, it causes a homologous recombination event at the loxP sites and removes the STOP cassette and turns on the expression of tdTomato turning the cells that turn red.