Notably, none of these CDK7 inhibitors led to any noticeable adverse effects or blood/liver toxicity in mice, consistent with previous research demonstrating that CDK7 is non-essential in adult tissues with low proliferative rates and that CDK7 loss primarily impacts highly proliferative cells.22 Samuraciclib is currently undergoing Phase I/II clinical trials for various tumor types, in particular estrogen receptor-positive breast cancer where it also shows acceptable safety profile with evidence of antitumor activity in combination with endocrine therapy.33 This evidence concerns the gene ESR1 and breast carcinoma.