A method of characterizing a library of a plurality of cell strains of different genotypes and having different variable regions in at least one part of the genetic material of said cell strains, said method comprises: loading cells of said cell strains with media into a fluid source of a microfluidic device comprising a substrate transparent for imaging and having a plurality of spatially defined and separated cell channels having a dimension to accommodate cells, wherein a respective first end of said plurality of spatially defined and separated cell channels is in fluid connection with a flow channel having said fluid source at a first end of said flow channel and a fluid sink at a second end of said flow channel and a respective second end of said plurality of spatially defined and separated cell channels is in fluid connection with a first end of a respective wash channel having a second end in fluid connection with a sink channel that is in fluid connection with a wash sink, wherein said wash channels have a dimension too small to accommodate cells; culturing cells of said cell strains at spatially defined and separated cell channels in said microfluidic device; determining a phenotypic characteristic of each cell strain in said spatially defined and separated cell channels in said microfluidic device; fixating said cells of said cell strains at said spatially defined and separated cell channels in said microfluidic device; in situ genotyping said variable region of each cell strain at said spatially defined and separated cell channels in said microfluidic device; and connecting each respective phenotypic characteristic to each respective genotype based on said spatially defined and separated cell channels in said microfluidic device. loading cells of said cell strains with media into a fluid source of a microfluidic device comprising a substrate transparent for imaging and having a plurality of spatially defined and separated cell channels having a dimension to accommodate cells, wherein a respective first end of said plurality of spatially defined and separated cell channels is in fluid connection with a flow channel having said fluid source at a first end of said flow channel and a fluid sink at a second end of said flow channel and a respective second end of said plurality of spatially defined and separated cell channels is in fluid connection with a first end of a respective wash channel having a second end in fluid connection with a sink channel that is in fluid connection with a wash sink, wherein said wash channels have a dimension too small to accommodate cells; culturing cells of said cell strains at spatially defined and separated cell channels in said microfluidic device; determining a phenotypic characteristic of each cell strain in said spatially defined and separated cell channels in said microfluidic device; fixating said cells of said cell strains at said spatially defined and separated cell channels in said microfluidic device; in situ genotyping said variable region of each cell strain at said spatially defined and separated cell channels in said microfluidic device; and connecting each respective phenotypic characteristic to each respective genotype based on said spatially defined and separated cell channels in said microfluidic device.