The first case study, CZS-035, is a ligand for polo-like kinase 4 (PLK4) and a warhead component used to synthesise a therapeutic PROTAC for breast cancer treatment, discovered by Sun et al. (Figure ). Both the baseline and mixed fine-tuned multi-step models successfully identify retrosynthetic routes for CZS-035 from purchasable precursors in our stock molecule database. Both models accurately reproduce the protection of nitrogen with Boc (A1) as seen in the literature synthesis. Both models also correctly identify the two S N Ar disconnections used in the literature to reproduce B1 and B2. However, the mixed fine-tuned model uniquely identifies the final ring disconnection of pyrazole in B1 to C1 and C2, which aligns with the literature approach. In contrast, the baseline model suggests the more complex and more expensive pyrazolopyrimidine C3 as the final purchasable precursor.