We have previously shown that several protein kinases are present in higher activity levels in multidrug resistant cell lines, such as KB-V1. We have now isolated a gene that codes for a putative protein kinase, PKY, of over 130 kDa that is expressed at higher levels in multidrug-resistant cells. RNA from KB-V1 multidrug-resistant cells was reverse-transcribed and amplified by using primers derived from consensus regions of serine threonine kinases and amplified fragments were used to recover overlapping clones from a KB-V1 cDNA library. An open reading frame of 3648 bp of DNA sequence predicting 1215 aa, has been identified. This cDNA hybridizes to a mRNA of about 7 kb which is expressed at high levels in human heart and muscle tissue and overexpressed in drug-resistant KB-V1 and HL60/ADR cells. Because its closest homolog is the yeast serine/threonine kinase, Yak1, we have called this gene PKY. PKY is also related to the protein kinase family that includes Cdks, Gsk-3, and MAPK proline-directed protein kinases. This protein represents the first of its type known in mammals and may be involved in growth control pathways similar to those described for Yak1, as well as possibly playing a role in multidrug resistance.