We report the observation of two pressure-induced quantum phase transitions in a YbB6 single crystal, i.e., from a topologically trivial semiconductor to an intermediate semimetal and then from the semimetal to a possible topologically nontrivial high-pressure gapped phase through in situ high-pressure transport and synchrotron x-ray diffraction measurements. Our high-pressure absorption results reveal that the mixed-valence state in pressurized YbB6 above 15 GPa plays an important role in the formation of the high-pressure state with a reopened gap. These high-pressure results may be helpful for shedding light on the intriguing relation between the topology and the 4f electrons in the rare-earth hexaborides.

Patient-derived xenografts (PDXs) are resected human tumors engrafted into mice for preclinical studies and therapeutic testing. It has been proposed that the mouse host affects tumor evolution during PDX engraftment and propagation, affecting the accuracy of PDX modeling of human cancer. Here, we exhaustively analyze copy number alterations (CNAs) in 1,451 PDX and matched patient tumor (PT) samples from 509 PDX models. CNA inferences based on DNA sequencing and microarray data displayed substantially higher resolution and dynamic range than gene expression-based inferences, and they also showed strong CNA conservation from PTs through late-passage PDXs. CNA recurrence analysis of 130 colorectal and breast PT/PDX-early/PDX-late trios confirmed high-resolution CNA retention. We observed no significant enrichment of cancer-related genes in PDX-specific CNAs across models. Moreover, CNA differences between patient and PDX tumors were comparable to variations in multiregion samples within patients. Our study demonstrates the lack of systematic copy number evolution driven by the PDX mouse host.