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Summary: There is an increasing need for tissue‐specific gene expression regulatory elements to study normal and disease development in the mouse. However, the cloning and characterization of these elements are time‐consuming and costly. Thus, there is a particular need to be able to identify gene expression patterns without having to clone the promoter elements. Gene‐trap strategies identify expression patterns assigned for endogenous genes using reporters, such as LacZ (Gossler et al., 1989; Skarnes, 1990) or green fluorescent protein (GFP) (Ishida and Leder, 1999; Zheng and Hughes, 1999). The gene‐trap vector randomly inserts into the genome and “steals” regulatory elements for the reporter. Here we describe an improved gene‐trap strategy, which allows an efficient Cre recombinase‐mediated insertion of any transgene into the trapped loci as a post‐integrational modification and links the expression of the transgene to that of the reporter. genesis 26:245–252, 2000. © 2000 Wiley‐Liss, Inc.

Radioligand assays have been used to study the production of insulin-like growth factor (IGF) and its carrier (IGFCP) by rat liver in organ culture. Bound IGF released into the culture medium was dissociated and