Scott's collections help reveal accelerating marine life growth in Antarctica

production of the selector itself — not initiation of its expression (Figure 1B). Initiation of terminal selector gene expression is, rather, triggered through transient regulatory inputs — signals and transcription factors that are only temporally present in a short window of time when a neuron is born. These transient inputs feed into the terminal selector locus via cis-regulatory elements that are separate from the autoregulatory elements that ensure maintenance after the initial ‘kick off’ phase (Figure 1A). The autoregulatory logic is thus a ‘lock-in’ or memory device that remembers a transient regulatory state. By current defi nition, a terminal selector therefore fulfi lls the function of a self-sustaining epigenetic trait that changes a cellular phenotype independent of alteration of genomic DNA sequence. As classic work has shown, transcriptional autoregulation and the resulting epigenetic maintenance and memory of a specifi c regulatory state is a deep evolutionary phenomenon, employed by systems as simple as bacteriophages. As transcriptional events usually lead to a characteristic pattern of histone modifi cations at the transcribed locus, terminal selector autoregulation and regulation of downstream terminal differentiation genes also involve histone-modifying enzymes. Yet the key specifi city determinant is the terminal selector transcription factor that selects target genes in a sequence-specifi c manner.