Science has chosen single-cell analyses of gene activity through time as its 2018 Breakthrough of the Year, honoring a trifecta of methods that together are enabling researchers to determine, at the individual cell level, which genes are turned on and off as an early embryo develops. "These technologies create some of the most extraordinary movies ever made, showing how a single cell grows into the intricate tissues and organs of a mature animal," says Tim Appenzeller, Science's news editor. The process involves isolating whole cells from organisms, sequencing their genomic contents in what is known as single-cell RNA-seq, and tagging early cells and their descendants to track how they split into multiple types during development. Scientists postulate single-cell RNA-seq could transform the basic biology and medical research landscape in the next ten years. "The ability to isolate thousands of individual cells and sequence each one's genetic material offers a snapshot of what RNA is being produced in each cell at that moment. Because RNA sequences are specific to the genes that produced them, researchers can immediately see which genes are active. These active genes define what a cell does," said Science staff writer Elizabeth Pennisi. "In 2018 alone, studies detailed how a flatworm, a fish, a frog, and other organisms begin to make organs and appendages. Groups around the world are applying the techniques to study how human cells mature over a lifetime, how tissues regenerate, and how cells change in diseases including cancer," Pennisi said. In a powerful complement to single-cell RNA seq, researchers have introduced molecular "trackers" (via fluorescent tags or the gene editing technique known as CRISPR) into early embryonic cells to mark them and track how they eventually become distinct cell lineages later in development. "By combining these techniques with single-cell RNA-sequencing, scientists can both monitor the behavior of individual cells and see how they fit into the unfolding architecture of the organism. Others are applying similar techniques to track what happens in developing organs, limbs, or other tissues - and how those processes can go wrong, resulting in malformations or disease," said Pennisi.