= C. Tuerk
{c}
= Craig Tuerk
{c}
{synonym}
\b[C. Tuerk], identified as \b[Craig Tuerk] in <L. Gold>'s firsthand account, coauthored the 1990 experiment establishing <SELEX>. The retrospective describes his investigation of the RNA binding site involved in regulation of a bacteriophage <DNA polymerase> and attributes the process name to him.<SELEX: How It Happened and Where It will Go by Larry Gold>[The First Moments]{ref} The fuller name is supported by that participant account; no middle name or life dates are inferred from the abbreviated research byline.
The primary paper selected <RNA> ligands after randomizing an eight-base region. There are $4^8=65{,}536$ possible strings for that region. Two recovered sequences differed at four positions yet had equivalent binding constants in the reported comparison.<Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase by C. Tuerk and L. Gold>[Abstract, sequence-pool size and binding-constant comparison]{ref} The observation distinguishes a difference in <nucleotide> sequence from a difference in the measured molecular function.
An original geometric count sharpens the scale of the experiment. Around one fixed eight-base string, the number of strings differing at exactly four positions is
$$
\binom84 3^4=70(81)=5670.
$$
The binomial coefficient chooses which four positions change; each chosen position has three alternative bases. A selected four-change sequence is consequently one of thousands at that same sequence distance. Sequence distance alone does not identify the functional substitute; the binding experiment supplies that information.
The theoretical count also does not prove that a physical preparation contains every possible sequence at equal abundance. Synthesis, survival during selection and subsequent <polymerase chain reaction> amplification can affect representation. Conversely, equivalent <binding affinity> to one target does not imply equal behavior toward every other target or equal stability in every environment.
Tuerk's contribution illustrates how a focused question about an <RNA> regulatory binding site could motivate a reusable experimental search procedure. A candidate's sequence can be recovered after selection and its binding measured separately, connecting the population-level search to an individually testable molecular result.
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