Abstract: The invention pertains to novel peptide analogs suitable for inhibiting protein:prenyl transferases. As such they are therapeutically useful in e.g. inhibiting oncogenesis and other unwanted cell proliferation, and in supressing aberrant high signal transduction. The analogs comply with the following formula: in which: R1 is hydrogen or a thiol-protecting group; R2 and R3 are independently hydrogen or C1-C4 alkyl; R4 is hydrogen, C1-C4 alkyl, C1-C4 acyl or peptidyl; R5 is hydrogen or C1-C4 alkyl; R6 is hydrogen or optionally substituted C1-C6 alkyl; A is a direct bond or an optionally substituted C1-C4 alkylene chain; Y represents an oxo group or two hydrogen atoms; Z is oxygen, sulphur, imino or C1-C5 alkyl-, aryl- or acylimino; M is 0, 1 or 2; N is 0 or 1.