An extensive study of analogs of the potent antitumor antibiotics CC-1065 and the duocarmycins which incorporate the 1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one (CBI) alkylation subunit are detailed. In contrast to early speculation, deep-seated modifications in the CC-1065 and duocarmycin alkylation subunits are well tolerated and the CBI-based analogs proved to be potent cytotoxic agents and efficacious antitumor compounds. Full details of studies defining a direct relationship between functional stability and in vitro cytotoxic potency are described. As such, the readily accessible CBI-based analogs were found to be four times more stable and four times more potent than the corresponding analogs containing the authentic CPI alkylation subunit of CC-1065 and comparable in potency to agents containing the authentic alkylation subunit of duocarmycin SA. Similarly, the CBI-based agents alkylate DNA with an unaltered sequence selectivity at an enhanced rate and with a greater efficiency than the corresponding CPI analog and were comparable to the corresponding analog incorporating the duocarmycin SA alkylation subunit. Systematic and extensive modifications and simplifications in the DNA binding subunits attached to CBI were explored with the comparisons of both enantiomers of CC-1065 and the duocarmycins 2 and 3 with enantiomers of 18-24, 25-29, 57-61, 62-65, 66-68, 72, 73, 78 and 79.