PURPOSE: To obtain human platelet factor 4 in improved expression and recovery of recombinant protein by selecting a DNA sequence having a specific length as the 1st DNA sequence of a hybrid gene.

CONSTITUTION: The objective process for improving the expression of a recombinant protein includes (1) a process for constructing a hybrid gene composed of the 1st DNA sequence coding amino terminal and separated with a cleavage site sequence and the 2nd DNA sequence coding the recombinant protein, (2) a process for culturing a host cell transformed with a vector having said hybrid gene and (3) a process for recovering fused protein from the transformed cell. The above process is carried out by the following manner. A recombinant protein is coded as the 1st DNA sequence of the hybrid gene. A DNA sequence having sufficient length to inhibit the formation of undesirable secondary structure from an mRNA region and an mRNA region containing a ribosome-linking site is selected to prevent the hindrance of the approach of ribosome to said ribosome-linking site.