  As the NRC report had recommended, priority at the outset of the project was given to mapping rather than sequencing the human genome. HGP scientists sought to construct two kinds of maps. Genetic maps order polymorphic markers linearly on chromosomes; the aim is to have these markers densely enough situated that linkage relations can be used to locate chromosomal regions containing genes of interest to researchers. Physical maps order collections (or “libraries”) of cloned DNA fragments that cover an organism's genome; these fragments can then be replicated in quantity for sequencing. The joint NIH-DOE five-year plan released in 1990 set specific benchmarks: a resolution of 2 to 5 centimorgans (cM) for genetic linkage maps and physical maps with sequence-tagged site (STS) markers (unique DNA sequences 100–200 base pairs long) spaced approximately 100 kilobases (kb) apart and 2-megabase (Mb) contiguous overlapping clones (“contigs”) assembled for large sections of the genome. Sequencing needed to be made more efficient and less costly: aims were to reduce sequencing costs to $.50 per base and to complete 10 million bases of contiguous DNA (0.3 percent of the human genome) but otherwise to focus efforts on the smaller genomes of less complex model organisms (Watson 1990). HGP goals were facilitated by a number of technological developments during this initial period. For physical mapping, yeast artificial chromosomes (YACs) introduced in 1987 (Burke et al. 1987) permitted much larger segments of DNA to be ordered and stored for sequencing than was possible with plasmid or cosmid libraries. A new class of genetic markers, microsatellite repeats, was identified in 1989 (Litt and Luty 1989; Tautz 1989; Weber and May 1989); because these sets of tandem repeats of short (either dinucleotide, trinucleotide, or tetranucleotide) DNA sequences are more highly polymorphic and detectable by PCR, microsatellites quickly replaced RFLPs as markers of choice for genetic linkage mapping and furnished the STS markers which facilitated the integration of genetic and physical maps. Another technological achievement—the combined use of reverse transcription, PCR, and automated sequencing to map expressed genes—led to administrative changes at the NIH when, in April 1992, Watson resigned from his position as director of the NCHGR following a conflict with NIH director Bernadine Healy over gene patenting. In 1991, while working at the NIH, J. Craig Venter sequenced small portions of cDNAs from existing libraries to provide identifying expressed sequence tags (ESTs) of 200–300 bases which he then compared to already identified genes from various species found in existing databases (Adams et al.  1991).[4] Watson disagreed with Healy's decision to approve patent applications for the ESTs despite lack of knowledge of their  function.[5] Soon after Watson's departure, Venter left NIH for the private  sector.[6]