Each tracer atom has been chemically incorporated into a biologically active molecule. The secondary metabolites are believed to be biologically and pharmacologically active. A biological system is a complex network of biologically relevant entities. Functional metagenomic studies enable the discovery of novel genes that encode biologically active molecules. The surface layers are often deficient in biologically active nitrogen compounds. Strategies include directly attaching photosensitisers to biologically active molecules such as antibodies. Echinoderms are important both biologically and geologically. Derivatives of quinones are common in biologically active molecules. Breast milk contains complex proteins, lipids, carbohydrates and other biologically active components. Monoglycerides are produced both biologically and industrially. Steroidogenesis entails processes by which cholesterol is converted to biologically active steroid hormones. The echinoderms are important both biologically and geologically. Stainless steel is generally considered to be biologically inert. They are the only forms of the molecule that are known to be biologically active. Several lncRNAs have been found to in fact encode for peptides with biologically significant function.