Oncogenes are a group of evolutionary conserved cellular genes (c-onc) homologous to the transforming genes of oncogenic retroviruses (v-onc). Some of them are localized near the breakpoints of specific chromosomal aberrations occurring in various neoplasms, as for example the Philadelphia translocation, t(9;22)(q34;q11), in chronic myelocytic leukemia (CML). Recently, we localized the human c-abl oncogene to chromosome region 9q34 and demonstrated a translocation of this gene to the Philadelphia chromosome (Ph1,22q-) in various forms of Ph1- positive, but not Ph1-negative, chronic myelocytic leukemia (CML). Another human oncogene, c-sis, is located on chromosome 22 and was recently reported to be transferred to chromosome 9q+ in one CML patient. We have now studied 2 CML patients with classic and variant types of Ph1 translocation, one Ph1-negative case, and a healthy control using in situ hybridization of a c-sis probe to metaphase chromosomes. These studies show that c-sis: (1) is localized to region 22q12.3-q13.1, far away from the breakpoint region 22q11 in CML, (2) segregates with the translocated part of chromosome 22 to different chromosomes in Ph1-positive patients, and (3) remains on chromosome 22 in the Ph1-negative case. Therefore, these data give no support for an active role of the c-sis gene in the generation of CML. Thus, if either of these two oncogenes is involved in the development of Ph1-positive CML, c-abl appears to be the more important one.