Steroid isolation , depending on context, is the isolation of chemical matter required for chemical structure elucidation, derivitzation or degradation chemistry, biological testing, and other research needs (generally milligrams to grams, but often more  or the isolation of "analytical quantities" of the substance of interest (where the focus is on identifying and quantifying the substance (for example, in biological tissue or fluid). The amount isolated depends on the analytical method, but is generally less than one microgram.  [ page needed ] The methods of isolation to achieve the two scales of product are distinct, but include extraction , precipitation, adsorption , chromatography , and crystallization . In both cases, the isolated substance is purified to chemical homogeneity; combined separation and analytical methods, such as LC-MS , are chosen to be "orthogonal"—achieving their separations based on distinct modes of interaction between substance and isolating matrix—to detect a single species in the pure sample. Structure determination refers to the methods to determine the chemical structure of an isolated pure steroid, using an evolving array of chemical and physical methods which have included NMR and small-molecule crystallography .  : 10–19 Methods of analysis overlap both of the above areas, emphasizing analytical methods to determining if a steroid is present in a mixture and determining its quantity. 
The drug-related mechanisms producing cumulative changes in neurotransmission sometimes are epigenetic in nature. While a drug cannot change a person’s genes, drugs can prod some genes to increase or decrease their production of proteins, leading to changes in neuron function or even actual reshaping of the physical structure of neurons. For example, in mice, cocaine alters important genetic transcription factors and the expression of hundreds of genes . Some of the resulting changes in the brain’s complement of proteins have been associated with increased drug-seeking and addiction-like behaviors in animals. Other changes, such as proliferation of new dendrites (branchlike structures on neurons that feature neurotransmitter receptors on their surface) may be compensatory . Some epigenetic changes can be passed down to the next generation, and one study found that the offspring of rats exposed to THC—the main psychotropic component of marijuana—have alterations in glutamate and cannabinoid receptor formation that affects their responses to heroin .