The monoclonal antibody Bx63 recognizes a centrosomal antigen of Drosophila melanogaster by indirect immunofluorescence and identifies two proteins, with apparent molecular weights of 185 x 10³ and 66 x 10³, on Western blots. We have used this antibody to isolate five clones (λcs1, -2, -3, -4 and λj63) from λgt11 expression libraries of Drosophila DNA. Using polyclonal anti-centrosomal sera raised against both immunoaffinity-purified Bx63 antigen and electrophoretically purified fusion protein from clone λcs3, we have demonstrated that the fusion proteins encoded by four of these clones (λcs1-4) share at least two epitopes with the 185 x 10³ M_r centrosomal antigen. This indicates that clones λcs1-4 contain DNA from the gene coding for this protein. The four clones are independent isolates from a single chromosomal site, which we show by in situ hybridization to correspond with salivary gland chromosome region 88E 4-8. A low-abundance transcript of approximately 4.0 x 10³ bases corresponding to the cloned gene is detected in all stages of the Drosophila life-cycle
