We have recently described a class of peptides that improve drug delivery by increasing penetration of drugs into solid tumors. These peptides contain a C-terminal C-end Rule (CendR) sequence motif (R/K)XX(R/K), which is responsible for cell internalization and tissue-penetration activity. Tumor-specific CendR peptides contain both a tumor-homing motif and a cryptic CendR motif that is proteolytically unmasked in tumor tissue. A previously described cyclic tumor-homing peptide, LyP-1 (sequence: CGNKRTRGC), contains a CendR element and is capable of tissue penetration. We use here the truncated form of LyP-1, in which the CendR motif is exposed (CGNKRTR; tLyP-1), and show that both LyP-1 and tLyP-1 internalize into cells through the neuropilin-1-dependent CendR internalization pathway. Moreover, we show that neuropilin-2 also binds tLyP-1 and that this binding equally activates the CendR pathway. Fluorescein-labeled tLyP-1 peptide and tLyP-1-conjugated nanoparticles show robust and selective homing to tumors, penetrating from the blood vessels into the tumor parenchyma. The truncated peptide is more potent in this regard than the parent peptide LyP-1. tLyP-1 furthermore improves extravasation of a co-injected nanoparticle into the tumor tissue. These properties make tLyP-1 a promising tool for targeted delivery of therapeutic and diagnostic agents to breast cancers and perhaps other types of tumors.