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Cell differentiation is the acquisition of a specialized cell state, involving changes in structure, function and gene expression. A neuron and a red blood cell illustrate different outcomes: signaling and oxygen transport. Different functions usually do not require a different inherited DNA sequence for each function. The shared-genome principle concerns nucleated precursors and most nucleated descendants: a mature mammalian red blood cell has lost its cell nucleus and mitochondria, so it is not an example of two mature biological cells both retaining the same nuclear DNA. ^[ref]

The puzzle is how a shared set of genes supports different jobs. Gene expression changes which RNA and proteins a biological cell produces, and in what amounts. A transcription factor can regulate several genes; different combinations of regulators can interpret the same signal differently. Stable regulatory patterns can persist through the cell cycle, providing memory of developmental history. Specialization therefore usually changes use of inherited information, rather than replacing the genome wholesale. ^[ref]

A small counting example separates sequence from activity. Suppose the same three toy genes are present in every nucleated biological cell, and independently assign each gene an on/off state. There are $2^3=8$ possible expression profiles with no DNA replacement. The figure shows only two: H and A on versus H and B on. Real gene expression is graded and coordinated, so this count illustrates possible descriptions, not a prediction of eight actual cell types. The interesting consequence is that a stable cell state can depend on how a shared record is used.

Stem cells combine self-renewal with production of differentiated descendants; progenitor cells generally have more restricted developmental options and renewal capacity. Neither term means that a biological cell must give rise to every cell type. Terminal differentiation denotes advanced specialization within a lineage, often accompanied by exit from the cell cycle. Cell fate concerns the developmental outcome of a biological cell or its descendants, whereas cell state describes its current properties. A temporary change in gene expression after signal transduction is not, by itself, proof of a new cell fate. ^[ref]

A lever is a body free to rotate about a fulcrum. Its usefulness comes from a balance of torques, not from producing energy. For a stationary ideal lever, let $d_E$ and $d_L$ be the perpendicular lever arms of the effort $E$ and load $L$. ThenThe mechanical advantage uses perpendicular distances to the lines of action, which need not equal distances measured along a bent or obliquely loaded arm. In an ideal slow motion, the effort moves through the larger distance when its force is the smaller: work is conserved.

For the three familiar straight arrangements, the fulcrum lies between effort and load in a first-class lever, the load lies between fulcrum and effort in a second-class lever, and the effort lies between fulcrum and load in a third-class lever. The second class has ideal mechanical advantage greater than one; the third has it less than one. The first can have either, depending on the lever arms.

The forearm bends by elbow flexion. The biceps brachii and brachialis pull through short tendons close to the elbow, while a load in the hand acts farther away. This is predominantly a third-class arrangement: a large muscle force buys a comparatively large and rapid hand displacement. The biceps brachii also contributes to forearm supination. Elbow extension is produced chiefly by the triceps brachii, whose tendon acts on the olecranon behind the elbow. The extensor insertion and forearm load lie on opposite sides of the effective pivot, giving the corresponding first-class analogy. Actual anatomical joint motion has changing muscle moment arms, and the joint reaction force also enters the full force balance; the body is not a set of frictionless pins. ^[ref]

A transcription factor is a protein that regulates transcription. Combinations of transcription factors help select gene expression patterns during cell differentiation. ^[ref]