KB-0742 (Kronos Bio) is a potent, orally bioavailable, selective inhibitor of CDK9 (cyclin-dependent kinase 9), the catalytic subunit of the Positive Transcription Elongation Factor b (P-TEFb) complex. CDK9 phosphorylates Ser2 of the RNA polymerase II C-terminal domain (CTD), releasing Pol II from promoter-proximal pausing and enabling productive elongation through gene bodies. At super-enhancers — transcriptionally hyperactive genomic regions defined by large clusters of H3K27ac, Mediator, and BRD4 occupancy — CDK9-mediated Pol II pause release is rate-limiting for high-level oncogene transcription. KB-0742 completed Phase 1 evaluation (NCT04718675; Kronos Bio; relapsed/refractory AML and MDS; Phase 1 monotherapy dose escalation and expansion), establishing clinical pharmacology, oral bioavailability, CDK9 target engagement, and a tolerability profile characterized by myelosuppression (mechanistically expected from transcriptional suppression of MCL1 and MYC in rapidly dividing hematopoietic progenitors). The SDH-deficient GIST rationale flows directly from the super-enhancer discovery of Merriam et al. (Nat Med 2026, PMID 42191879). SDH loss → succinate accumulation → competitive inhibition of α-KG-dependent TET1/2/3 5-methylcytosine dioxygenases → CIMP (global CpG island hypermethylation; Letouzé et al. Cancer Cell 2013, PMID 23550148; Killian et al. Cancer Cell 2013, PMID 23707781) → aberrant methylation of CTCF insulator sequences at chromatin domain boundaries → disruption of topological associating domain (TAD) boundaries → ectopic chromatin loop formation → activation of a de novo FGF3/FGF4 super-enhancer that is absent in KIT/PDGFRA-mutant GIST and normal adult mesenchymal tissue. This SDH-deficient-GIST-specific super-enhancer drives high-level FGF3/FGF4 transcription — a direct oncogenic output of the CIMP-driven chromatin remodeling cascade. Active super-enhancers universally depend on CDK9/P-TEFb for Pol II pause release and elongation. BRD4 binds H3K27ac marks at the ectopic SDH-GIST super-enhancer via its bromodomains and recruits P-TEFb (CDK9 + Cyclin T1) via the BRD4 C-terminal interaction domain on Cyclin T1. CDK9 then phosphorylates Pol II CTD Ser2, enabling elongation through FGF3/FGF4 gene bodies and other SE-activated loci. CDK9 inhibition by KB-0742 selectively suppresses super-enhancer-driven transcription at this SDH-deficient-GIST-specific SE — a mechanistically distinct intervention from BRD4 inhibitors (which block BRD4 chromatin reading upstream) using an orthogonal drug target (kinase vs. bromodomain reader). The BRD4/BET inhibitor direction is already in this engine. CDK9 inhibition acts at the same super-enhancer axis but one step downstream: BRD4 reads H3K27ac (inhibited by BET inhibitors); BRD4-recruited CDK9 phosphorylates Pol II CTD Ser2 (inhibited by KB-0742). The two targets are mechanistically non-redundant — CDK9 inhibition bypasses BRD4 entirely and directly blocks elongation kinase activity regardless of BRD4 occupancy — making CDK9 inhibition an independent therapeutic strategy and a potential combination partner with BET inhibitors on the same SE axis. Key limitations: (1) No published experimental data test KB-0742 or any CDK9 inhibitor in SDH-deficient GIST cell lines, animal models, or patient-derived xenografts. (2) The super-enhancer biology anchor (PMID 42191879) is directly established in SDH-deficient GIST, but CDK9 dependency of the ectopic FGF3/FGF4 SE specifically — vs. other transcription elongation kinases — has not been directly tested. (3) KB-0742 Phase 1 (NCT04718675) was conducted in hematologic malignancies (AML/MDS) with SE-dependent MYC/MCL1 transcription; GIST-specific pharmacology and dosing are unknown. Required next steps: CDK9 inhibitor dose-response curves in SDHA-null (e.g., GIST48) vs. SDH-intact GIST cell lines; Pol II CTD Ser2 phosphorylation as pharmacodynamic readout; FGF3/FGF4 mRNA levels as SE-specific transcriptional output; rescue by FGF3/FGF4 recombinant protein or CDK9 overexpression as mechanistic controls. Evidence_score 27 (theoretical): the mechanistic chain from SDH loss through CIMP, CTCF disruption, SE formation, and CDK9 elongation dependency is biologically coherent and each individual step is experimentally established. The SDH-deficient GIST super-enhancer is directly demonstrated (PMID 42191879), but CDK9-specific dependency of this SE has not been tested. KB-0742 provides a clinical-grade CDK9 inhibitor with established human pharmacology, grounding the pharmacological hypothesis.