One synonym for "highly homologous gene" is "closely related gene." This suggests that two genes share a significant amount of similarity in their nucleotide sequence or protein structure. Another synonym is "orthologous gene," which implies that the gene evolved from a common ancestor and maintains similar function and sequence in different species. "Paralogous gene" is another synonym that refers to genes within the same species that have evolved from a gene duplication event and may have similar or divergent functions. Overall, these synonyms highlight the importance of understanding the genetic relationships between different organisms and within species.