The present invention provides peptides comprising a sequence of X?6X?5X?4X?3X?2X?1X1PX3X4PX6X7PGX10X11AX13X14X15X16LX18X19X20X21X22X23LX25X26 YLX29X30X31X32 (SEQ ID NO : 13) wherein the amino acids X?6, X?5, X?4, X?3, X?2, X?1, X1, X3, X4, X6, X7, X10, X11, X13, X14, X15, X16, X18, X19, X20, X21, X22, X25, X26, X29, X30, X31, and X32 are as defined herein. The present invention further provides pharmaceutical compositions comprising the peptides and methods of using the peptides for treating proliferative diseases such as cancer which are associated with Ras. Also provided are methods of screening a library of peptide dimers using a peptide dimer display technology.