Hi Mădălin, > * gnu/packages/bioinformatics.scm (fast-screen): New variable. I don’t think this package actually produces a usable output. Frustratingly, this is a Perl script which calls out to tools that happen to be on the user’s PATH, such as Bismark (which is written in the same style, so it may be enlightening to read its package definition), bwa, or bowtie. Simply copying the script to the store won’t yield a usable tool I’m afraid. For Bismark I talked to the authors in the past in the hopes of simplifying configuration at build time, but they were not interested in changing the tool to accomodate any other case than the one Bismark was designed for: to be unpacked in an already suitable environment. This means that we can’t count on upstream to change this and patch the source file by ourselves. One way is to patch every invocation of an external command; another is to wrap the script itself (with “wrap-script”) in PATH and PERL5PATH to provide a suitable environment at runtime. (Wrapping PERL5PATH may be necessary anyway to ensure that Perl can find the required modules.) Good luck! -- Ricardo