The orthorhombic low-temperature phase of uranium metal. Its anisotropic thermal and irradiation response makes crystal grain crystallographic texture important for dimensional stability.

A ubiquitin-activating enzyme, or E1, uses ATP to activate ubiquitin and forms a covalent intermediate through a catalytic cysteine. Ubiquitin is then passed to a ubiquitin-conjugating enzyme. ATP consumption occurs at this activation stage, while the downstream relay handles targeting and transfer. This creates a chemically activated donor for controlled covalent protein modification. ^[ref]

A ubiquitin-conjugating enzyme, or E2, accepts activated ubiquitin from a ubiquitin-activating enzyme onto a catalytic cysteine. It then works with a ubiquitin ligase to transfer ubiquitin onward. Pairing between E2 and E3 helps determine which modification is assembled. This division of labour separates initial energy-dependent activation from later substrate selection and transfer. ^[ref]

A ubiquitin ligase, or E3, promotes transfer of ubiquitin to a selected substrate within the ubiquitination system. Many E3s provide substrate recognition, while the detailed chemistry differs between E3 families. A ubiquitin-conjugating enzyme supplies activated ubiquitin. The resulting modification can signal protein degradation or other fates depending on substrate, linkage and context, so ubiquitination is not synonymous with immediate destruction. ^[ref]

A microRNA whose endogenous activity restrains tumour development or progression in a specified context. Specific loss, controlled restoration and target tests provide stronger evidence than an expression correlation.

A leaf of a tree is a vertex of degree one in an unrooted tree. In a rooted tree, the usual convention instead calls a vertex a leaf of a tree when it has no children, so the one-vertex rooted tree has a leaf even though its degree is zero. Every finite tree with at least two vertices has at least two leaves: the endpoints of a longest simple path cannot have an unused neighbour without extending that path. Removing a leaf and its incident edge leaves a tree, a useful step in mathematical induction.

For finite nonempty trees, the dissymmetry theorem for trees gives an isomorphism of combinatorial classeswhere the superscripts mark a vertex, an undirected edge, or a directed edge. The tree on each side retains the same labels or other decorations; the identity is stronger than a count of labeled trees.

Repeatedly remove all leaves until one vertex or one edge remains. This is the invariant center of a tree. If a marked vertex or edge is the center, forget the mark and obtain the unrooted tree. Otherwise use the unique path toward the center: a marked vertex determines the first edge oriented toward the center, while a marked edge is oriented away from the center. These assignments give all directed-edge marks exactly once and can be reversed from the center and the orientation. This supplies the required bijection.

For a fixed tree with $n$ distinctly labeled vertices, the counts reduce to $1+2(n-1)=n+(n-1)$. For unlabeled trees, symmetry makes the bijection essential: one cannot simply divide a rooted count by $n$.

A lobster tree is a tree whose removal of all leaves leaves a caterpillar tree.

A caterpillar tree is a tree whose non-leaf vertices lie on a single path, called its spine. Equivalently, deleting all degree-one vertices leaves a path, a single vertex, or an empty graph in the smallest cases. A star graph is a caterpillar tree with a one-vertex spine; every path graph is also a caterpillar tree. Trees with branching that persists after deleting the leaves fail the definition. The spine packages a seemingly branching structure into a linear backbone with attached leaves, which simplifies many counting and layout questions.

A plane tree is a tree embedded in an oriented plane, with the embedding considered up to deformation preserving the cyclic order of incident edges. The cyclic order distinguishes plane embeddings that an abstract graph isomorphism can forget.

After choosing a root, the parent edge gives a starting position for the children at each nonroot vertex. At the root there is no parent edge: a marked root corner is needed to turn its cyclic order into a linear child order. A rooted plane tree includes that extra choice, equivalently an ordered child list at every vertex. Merely distinguishing the root vertex does not select its first child.

A solitary wave is a spatially localized travelling wave that retains its profile as it propagates. The displayed form uses a zero background; other constant backgrounds are possible. A soliton additionally has the characteristic elastic interaction property with other solitons. The existence of a single localized travelling profile does not alone establish that stronger property.

A carrier response must appear once in an electromagnetic constitutive model. With harmonic convention $e^{-i\omega t}$, absolute permittivity $\epsilon$ describing the remaining polarization and an explicit conduction electric current density ${\mathbf{J}}_c=\sigma \mathbf{E}$, the Ampère-Maxwell equation containsThus an alternative description incorporates that carrier contribution into the complex permittivity ${\epsilon }_{\mathrm{eff}}=\epsilon +i\sigma /\omega$ and omits the explicit ${\mathbf{J}}_c$. Using this effective quantity and adding the same conduction current again doubles that response. For relative permittivity, the added term is $i\sigma /({\epsilon }_0\omega )$; changing the harmonic sign convention changes the displayed sign. Stating which charges are included in each term prevents a mathematical rearrangement from becoming an extra physical mechanism.

A membrane-enclosed particle released by a cell into the extracellular environment. Isolation methods and cargo characterisation determine what signalling or transfer claims can be made.

Formation of a sample-spanning conducting path in a network of metal wires. Connectivity depends on wire number density, aspect ratio, orientation and the sample's finite dimensions; resistance also depends on the quality and number of wire junctions. A barely spanning network can have poor reliability even when individual wire resistivity is low.

A historical metaphor for a predicted but unsampled evolutionary connection. Modern phylogenetic hypotheses concern branching relationships and character changes rather than one obligatory intermediate.