FOXO4

FOXO4-DRI is a forty-six-residue synthetic peptide built entirely from D-amino acids in reversed sequence order, fused to a cell-penetrating segment. It is not the protein whose name it borrows.

That distinction is the first thing to be clear about. FOXO4 is a human transcription factor of five hundred and five residues, UniProt P98177. The material sold at this scale is the retro-inverso peptide described by Baar and colleagues in Cell in 2017, which is a different molecule entirely; any specification carrying the transcription factor's residue count or mass is describing something that does not come in a vial.

Thirty-three of its residues correspond to residues 92 to 124 of P98177, spanning the disordered region into the first helix of the forkhead domain. The remaining thirteen are the HIV-1 Tat basic segment together with a proline triplet, which Baar and colleagues describe as included to carry the peptide across the membrane. Because the whole chain is reversed, that Tat segment sits at the C-terminal end of the peptide as written, not the N-terminal end - a frequent annotation error.

Retro-inversion is the design. Reversing the backbone while inverting the configuration at every alpha carbon leaves the side chains in approximately their original spatial arrangement while removing the chain as a substrate for proteases that recognise L-peptides. The mass of 5358.2 is the figure stated in the paper's methods section and it is for the free peptide with a free amine and a free acid; there is no acetylation, no amidation, no cysteine and no disulfide.

No CAS number and no molecular formula are printed here because none exists. This peptide is in the primary literature but has never been entered into a chemical registry: there is no PubChem compound record for the assembled forty-six-mer and no FDA UNII, so there is no identifier a buyer can match a certificate against.

One consequence of that follows directly and is worth saying rather than leaving implicit. The all-D peptide and the ordinary L-peptide of the same sequence have identical masses and are indistinguishable by mass spectrometry; separating them needs a chiral method such as amino-acid analysis with Marfey's reagent, optical rotation or circular dichroism. A certificate reporting mass and chromatographic purity alone does not establish that the material is the D-form.

The peptide has one tryptophan and one tyrosine, so absorbance at 280 nm is usable, and D-configuration does not change ultraviolet absorbance. Its characterised molecular partner is the disordered transactivation domain of p53 (UniProt P04637), measured by isothermal titration calorimetry and NMR in a 2025 Nature Communications study, where it competes with the interaction the parent protein makes.