Updated project metadata. Lysine and arginine methylation are amongst the most frequent modifications on unstructured histone tails and in combination with other modifications provide the basis for a combinatorial 'chromatin or histone code'. Recognition of modified histone residues is accomplished in a specific manner by 'reader' domains that recognize chromatin modifications allowing to associate with specific effector complexes mediating chromatin functions. The methyl-lysine and methyl-arginine reader domain protein SPINDLIN1 (SPIN1) belongs to a family of 5 human genes, and has been identified as a putative oncogene and transcriptional co-activator containing three Tudor domains, able to mediate chromatin binding. Here we report on the discovery of the potent and selective bivalent Tudor domain inhibitor VinSPINIn* (see Footnote), which simultaneously engages Tudor domains 1 and 2 and effectively competes with chromatin binding. Inhibitor, chemoproteomic and knockdown studies in squamous cell carcinoma suggest an un-anticipated complexity of SPIN isoform mediated interactions in regulating cellular phenotypes.