Ultrasound is among the most widely used biomedical imaging modalities, but has 
limited ability to image specific molecular targets due to the lack of suitable 
nanoscale contrast agents. Gas vesicles-genetically encoded protein 
nanostructures isolated from buoyant photosynthetic microbes-have recently been 
identified as nanoscale reporters for ultrasound. Their unique physical 
properties give gas vesicles significant advantages over conventional 
microbubble contrast agents, including nanoscale dimensions and inherent 
physical stability. Furthermore, as a genetically encoded material, gas vesicles 
present the possibility that the nanoscale mechanical, acoustic, and targeting 
properties of an imaging agent can be engineered at the level of its constituent 
proteins. Here, we demonstrate that genetic engineering of gas vesicles results 
in nanostructures with new mechanical, acoustic, surface, and functional 
properties to enable harmonic, multiplexed, and multimodal ultrasound imaging as 
well as cell-specific molecular targeting. These results establish a 
biomolecular platform for the engineering of acoustic nanomaterials.