Identification of the natural, endogenous peptides encoded by lncRNA ORFs using mass spectrometry (MS). (A) The unique peptide of TP53LC04 was identified using shotgun MS in HepG2-ADR⁺ cells. (B,C) The unique peptides of TP53LC02 (B) and TP53LC04 (C) were identified using PRM MS in HepG2-ADR⁺ cells. (D–F) The unique peptides of TP53LC02 (D), TP53LC03 (E), and TP53LC04 (F) were identified using multiple reaction monitoring MS (MRM-MS) in HepG2-ADR⁺ cells. Mass chromatograms of the corresponding endogenous peptides are showed on the left, and the corresponding heavy-peptides are indicated on the right. (G) Immunoblotting validation of antibodies against TP53LC04. The indicated protein levels of cells overexpressing TP53LC04, and stable sh-TP53LC04 cells treated with ADR are shown.