The present invention relates to nucleotide sequences of TCL-1 genes and amino acid sequences of their encoded proteins, as well as derivatives and analogs thereof, and antibodies thereto. The TCL-1 gene sequence is preferentially expressed early in T and B lymphocyte differentiation. The present invention further relates to the use of TCL-1 genes and their encoded proteins as diagnostic and therapeutic reagents for the detection and treatment of disease states associated with chromosomal abnormalities.