Complementary DNA encoding a catalytic subunit of protein phosphatase 1, termed PP1 beta, was isolated from a human teratocarcinoma library. Hybridisation with different cDNA fragments showed that all human tissues examined contained 3.1 kb, 4.0 kb and 5.4 kb PP1 beta mRNAs arising from alternative splicing of the 3' noncoding region. The level of the 5.4 kb mRNA relative to the 3.1 kb mRNA was higher in skeletal muscle than in other tissues and the PP1 beta/PP1 alpha mRNA ratio in rabbit tissues was highest in skeletal muscle. The 3' noncoding region of PP1 beta showed extreme conservation (> or = 90% identity) between man and rodents over 1.7 kb, suggesting that this region is of functional importance. The gene for human PP1 beta (PPP1CB) was localised to chromosome 2 by analysis of somatic cell hybrid DNA and mapped to band q23 by fluorescence in situ hybridization. These data show that the genes for three protein phosphatase catalytic subunits PP1 alpha, PP1 beta, PP1 gamma are all located on different chromosomes.