Simple SummaryThe aim of the study was to detect the nucleotide sequence variability in the calcium/calmodulin-dependent serine kinase (CASK) gene in pigeons. One of the detected polymorphic sites (g.8893G > A) located at position −3 relative to the start codon was selected for genotyping in 517 pigeons and was used to establish possible associations between genotypes and flight performance of pigeons. Frequencies of theAAgenotype were higher in homing pigeon groups than in the group of non-homing pigeons. The results can be treated as a contribution to further research that could confirm the functionality of the analysed SNP in shaping the sports phenotype of domestic pigeons.AbstractCalcium/calmodulin-dependent serine protein kinase (CASK) is an multidomain protein involved in tissue development and cell signalling. In skeletal muscle, it is involved in the development of neuromuscular junctions. The participation of a pigeon in racing is a great physical effort that causes many changes in the skeletal muscles. Thus, the purpose of the study was to detect the nucleotide sequence variability in the calcium/calmodulin-dependent serine kinase (CASK) gene in domestic pigeons (Columba livia domestica)and assess the potential impact of DNA polymorphisms on the flight performance of pigeons. The research included a total of 517 individuals. DNA was extracted from the blood. A DNA fragment from nucleotides 8689 to 9049 of theCASK(NW_004973256.1 sequence) of six unrelated pigeons were sequenced. One of the detected polymorphic sites (g.8893G > A), located a very close to the start codon, was selected for genotyping in all individuals. The association studies included a total of 311 young homing pigeons that participated in racing competitions. The homing pigeons showed higher frequencies of theAAgenotype than non-homing ones (p< 0.05). In rock pigeons only theGGgenotype was found. Further research could confirm the functionality of theCASKg.8893G > A SNP in shaping the racing phenotype of pigeons, and theAAgenotype could be useful as a selection criterion in pigeon breeding.