eturn a list of websites that were used as references for
Drugs/ Carriers/ Enzymes/ Targets/ Transporters</p>
    </div>

    <div id="ref-usage">
    <div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">drugs_links</span><span class="op">(</span><span class="op">)</span></span>
<span></span>
<span><span class="fu">carriers_links</span><span class="op">(</span><span class="op">)</span></span>
<span></span>
<span><span class="fu">enzymes_links</span><span class="op">(</span><span cl