Kimura and Crow’s 1964 article is justly regarded as one of the foundational articles of evolutionary molecular genetics. It is based on what they called the “infinite alleles” model (now called the “infinitely many alleles” model). This model was motivated by the recognition that a gene is a sequence of perhaps several thousand nucleotides, implying that an astronomically large number of alleles (equal to nucleotide sequences) is possible at any locus. In the later sections of their article Kimura and Crow discuss aspects of this model when selection exists. However, the theory for this case is still unresolved and here only the first section, which in any event has been by far the most influential section, is discussed. In that section only selectively neutral alleles (“isoalleles”) were considered.