N3-PEG3-CH2CH2COOH, a PEG-based PROTAC linker, is utilized in the synthesis of BI-3663, BI-4216, and BI-0319. Additionally, Azido-PEG3-acid, a non-cleavable 3-unit PEG ADC linker, is employed in the synthesis of antibody-drug conjugates (ADCs).