We have identified a novel protein kinase-encoding gene, KIN3, in the genome of the budding yeast Saccharomyces cerevisiae. The gene was isolated from a library of cloned genomic fragments by probing with an oligodeoxyribonucleotide mixture corresponding to part of a highly-conserved region in the catalytic domain of protein serine-threonine kinases. KIN3 is unique in the yeast genome, maps to chromosome VI and is actively expressed in mitotically dividing cells to produce a 1400 nucleotide (nt) message. The nt sequence of KIN3 predicts a protein product of 43.4 kDa which contains all of the conserved elements found in known protein serine-threonine kinases, although the organisation of these elements in the KIN3 gene product differs significantly from the consensus. The function of the KIN3-encoded protein kinase is unclear although it appears not to be essential for growth, conjugation or sporulation.