JAK1A is a recently isolated class 3 (nonreceptor) tyrosine kinase that has catalytic domain sequence homology with other kinases and is known to be ubiquitously expressed in all human tissues thus far examined. The gene for this enzyme had previously been localized to chromosome 1 using somatic cell hybrids and linkage analyses. In the present study, fluorescence in situ hybridization was utilized to confirm its localization and regionally assign the gene to chromosome region 1p32.3-->p31.3.